In the book of Matthew, Jesus is approached by the Pharisees. The Pharisees present Jesus with one of the contentious issues of the day: divorce. Jesus questions their fundamental understanding of marriage. To Jesus, when people are joined, they are joined by God and there is no return:

Mat 19:4 He answered, “Have you not read that he who created them from the beginning made them male and female,
Mat 19:5 and said, ‘Therefore a man shall leave his father and his mother and hold fast to his wife, and the two shall become one flesh’?
Mat 19:6 So they are no longer two but one flesh. What therefore God has joined together, let not man separate.”

One thing to note is that God joined the individuals. It is “man” that is separating what God joined. In Jesus’ theology, is this possible? Or is this Jesus’ way of saying that man cannot undo what God has done even if they do so socially (it would be a rhetorical point)? Jesus states, later in the same passage, that remarriage after divorce is still adultery. In the theology of Jesus, divorce is a man-made concept (not recognized by God).

Jesus states that the only real divorceable offense is adultery:

Mat 19:9 And I say to you: whoever divorces his wife, except for sexual immorality, and marries another, commits adultery.”

Jesus’ logic flows from sex being the uniting act. Once a woman is joined to a man then that bond is sealed eternally. If a man or woman forms that bond with another person, the unique bond is no longer unique; it has become common and vulgarized. Divorce for adultery does not lead to adultery because once adultery is committed, the bond is already broken.

This was a two way street. Both men and women could break the covenant. In Jesus’s understanding, activities like polygamy were a violation of marriage. Because marriage was a bond between two people, any deviation was suspect. Notice the parallel passage in Mark:

Mar 10:11 So He said to them, “Whoever divorces his wife and marries another commits adultery against her.
Mar 10:12 And if a woman divorces her husband and marries another, she commits adultery.”

In this scenario, adultery occurs after the divorce is finalized (the idea is that a social divorce may be state recognized but is not recognized by God). The adultery occurs, then, once the divorced individuals remarry. Theoretically, if the individuals do not remarry (have sex with other individuals), no adultery occurs.

In the mindset of Jesus, marriage centered around the act of sex. Sexual union constituted spiritual union. Once the circle has been expanded from husband and wife, that circle is broken. There are no marital bonds left in an adulterous marriage.

When Jesus’ quotes the Old Testament, this quote is coming directly from Genesis:

Gen 2:24 Therefore a man shall leave his father and his mother and hold fast to his wife, and they shall become one flesh.

Jesus’ concept of sex uniting body and soul is a fundamental aspect of God’s original creation. God creates Adam to be the perfect being, and for Adam he develops a helper (Eve). The two engage in sexual relations, and the two become one. The marriage act (sex) is a merging of two individuals.

This is God’s intended standard. There is no adultery, or polygamy, or multiple sexual partners. Instead, there are two individuals united in marriage. This was the intended creation, although this is not how society eventually functioned (as seen through the laws of ancient Israel).

In keeping with this understanding of marriage, Jesus uses some very interesting terminology in Matthew 5:

Mat 5:32 But I say to you that whoever divorces his wife for any reason except sexual immorality causes her to commit adultery; and whoever marries a woman who is divorced commits adultery.

Jesus states that the act of divorce “causes” a woman to commit adultery. Jesus is not blaming the woman for being an adulterer. In this view, her remarriage was only the logical (and acceptable) choice. But because of the logistics as to how marriage works, this act (although acceptable), is adultery nonetheless. The blame is being placed on the man, who is culpable of making his wife an adulterer.

Paul held identical beliefs about the function of sex and marriage. Paul reinforces this concept in his works. In Corinthians, Paul criticizes Christians who use prostitutes. In doing so, he explains how prostitutes vulgarize the bond of marriage:

1Co 6:15 Do you not know that your bodies are members of Christ? Shall I then take the members of Christ and make them members of a prostitute? Never!
1Co 6:16 Or do you not know that he who is joined to a prostitute becomes one body with her? For, as it is written, “The two will become one flesh.”

Notice, much like the implication from Jesus and from Genesis, Paul is equating the sexual copulation with “becoming one”. In the theology of Paul, the sex act unites individuals. Paul’s argument is that by having sex with prostitutes, Christians are forming a marriage covenant with those same prostitutes. This in turn makes prostitutes (which the text considers dirty) part of the church (body of Christ).

Sex constitutes the foundation of marriage.

The Ideal

In Jewish theology, God has established an ideal plan for the world. God has His views on how the world should operate and how individuals should act. Deviation from this ideal distanced individuals from God.

In God’s perfect world, individuals would not die, individuals would marry as virgins, individuals would not suffer deformity, there would be no disease. Man was to live perfect and healthy lives. As such, God sought to use the priestly caste, the caste closest to Himself, to mimic this perfection. He imposed upon the priests special rules and commands that applied only to themselves.

The High Priest was not allowed to scar his body or even have natural deformities. He could not be maimed in battle, but had to have physical perfection. The High Priest could not attend funerals (or be in contact with death). The High Priest was not allowed tattoos, and his daughters could not be prostitutes (under penalty of death). He could not be born of a forbidden union (most likely of adultery or whoredom). If any priest came in contact with disease, they were unclean. If his daughter was married to a non-priest, then she was no longer considered family and could not eat the Holy food (note that this dispossession of a daughter in marriage affirms the Genesis description in which the married couple merges).

But very interestingly, the priests were also imposed a standard of virginity in marriage:

Lev 21:13 And he shall take a wife in her virginity.
Lev 21:14 A widow, or a divorced woman, or a woman who has been defiled, or a prostitute, these he shall not marry. But he shall take as his wife a virgin of his own people,
Lev 21:15 that he may not profane his offspring among his people, for I am the LORD who sanctifies him.

Any woman who had ever had sex, even in the bounds of marriage, was off-limits to marrying the priestly class. This makes sense in the context in which sex equals an eternal bond. If ever a woman had sex, she was joined to her partner, and marrying another constituted an unideal arrangement (a deviation from God’s standard). This concept was re-enforced such that even if a priest’s sister died, the priest could not attend her funeral unless she was a virgin (although allowances were made for widows to reintergrate into the family, possibly for reasons of practicality). This image of marriage was God’s standard.

Deviations

While God’s standard is one man, one woman, one marriage, there were many deviations from this standard. Most of these deviations were instituted for practical reasons.

Normal Israelites (not of the priest class) were allowed to marry impure women (widows, prostitutes, and promiscuous women). Remarriage of a woman after her husband’s death was encouraged and sometimes mandated. The penalty for a brother not furthering the lineage of a deceased sibling was that his house would forever be labeled “The house of him who had his sandal pulled off”:

Deu 25:5 “If brothers dwell together, and one of them dies and has no son, the wife of the dead man shall not be married outside the family to a stranger. Her husband’s brother shall go in to her and take her as his wife and perform the duty of a husband’s brother to her.
Deu 25:6 And the first son whom she bears shall succeed to the name of his dead brother, that his name may not be blotted out of Israel.
Deu 25:7 And if the man does not wish to take his brother’s wife, then his brother’s wife shall go up to the gate to the elders and say, ‘My husband’s brother refuses to perpetuate his brother’s name in Israel; he will not perform the duty of a husband’s brother to me.’
Deu 25:8 Then the elders of his city shall call him and speak to him, and if he persists, saying, ‘I do not wish to take her,’
Deu 25:9 then his brother’s wife shall go up to him in the presence of the elders and pull his sandal off his foot and spit in his face. And she shall answer and say, ‘So shall it be done to the man who does not build up his brother’s house.’
Deu 25:10 And the name of his house shall be called in Israel, ‘The house of him who had his sandal pulled off.’

In this text, a brother (without children) dies. The duty is passed to the second brother to give children to the wife who had none. If he refuses this duty, he is publically mocked and scorned. For his lack of concern for his brother’s lineage, a curse was placed on his house such that he too is cut off from having children.

In God’s perfect plan, brothers have children and do not die. But because God’s plans are not realized on Earth, sometimes brothers die childless. The circumstances may demand odd actions to rectify the situation. In God’s standard, per the logic of the priestly code, the widowed woman has already formed her bond. She was off limits for further sex with other individuals. But per lineage concerns, she married within the family. She was also free to pursue alternative marriages if a brother refused to fulfill his duties.

Marriage after the death of a spouse was common. One of the most famous examples is that of Ruth, who then becomes included in the lineage of Jesus (much like another odd figure, the prostitute, Rehab). Normal Israelites would marry divorcees, promiscuous women, and sometimes even prostitutes (such as in the case of Hosea). Such were shameful things, and a high price was placed on virginity.

Polygamy was also legal, although a deviation from the ideal. Michael Heiser writes:

So, does the Old Testament “approve” of polygamy? Yes, in the sense that (a) it was part of the culture at the time God chose to call Abraham and create a people through him and his wife Sarah and (b) God didn’t care to outlaw the culture of the time. But it would be misguided to think that God promoted polygamy or held it out as the most desirable option. The Old Testament holds monogamy as an ideal, and makes no effort to argue that polygamy was a desirable situation for men in general. Polygamy just “was” and God didn’t care about the culture in which he initiated the next phase of his salvation plan. Polygamy had no vital theological place in that plan and would ultimately become even culturally irrelevant when Israel was replaced as the circumcision-neutral Church as the people of God. 1 It just wasn’t an issue.

Polygamy was allowed, and there is at least one example of polygamy even among the priest caste: Elkanah, father of Samuel (although the circumstances of his polygamy are not explored in the text).

Paul shows his disdain for polygamy through his qualifications for being an elder of the church:

1Ti 3:2 Therefore an overseer must be above reproach, the husband of one wife, sober-minded, self-controlled, respectable, hospitable, able to teach,

There has been discussion about what Paul means here. He may be saying: Elders have to be married, or elders cannot have divorced, or even elders cannot be polygamous. If Paul is saying elders cannot have been divorced and remarried, this would also seem to cover polygamous relationships. Paul’s point would be an affirmation of Jesus’ concept of how marriage works, and this would be consistent to his own treatment of the marital relationship.

Although Paul does not outlaw polygamous and/or remarried elders within the church ranks, it is not allowed to infiltrate the ranks of church leadership. It is tolerated but disdained. Much like the priests, the elders of the church were held to a higher standard, and had to lead more perfect lives (setting the standard by which others must follow).

Criminalized Activity

While certain sexual activities were not criminalized (such as polygamy, prostitution, and general promiscuity) plenty of sex acts incurred very extreme punishment. Death was given to rapists, adulterers (even those who have sex with engaged/betrothed women), those who have sex with both mother and daughter, those who have sex with their close relatives (following the idea that sex bonds, death was given for having sex with both blood relatives and relatives by marriage), homosexuality, bestiality, those who have sex with menstruating women, and even prostitutes who are the daughters of priests (per the priestly purity laws).

To opine that the Bible is not a thorough book and cannot cover all sex scenarios (thus does not cover prostitution or polygamy or promiscuity) is not a very good argument. How common is prostitution compared to men who have sex with menstruating women? Compared to bestiality? Compared to sex with a both a daughter and mother? Would not a prohibition against sex with multiple people cover sex with both a mother and daughter? Why would the author not save some time and just illegalize all extra-marital sex? The fact is that American morality seems to diverge from Israelite morality.

The modern American might not think twice about some of these sexual relations that God condemns with death. It is near impossible to figure out which men are having sex with menstruating women, and Americans would be hard pressed to even hold a discussion the issue. But, in the Bible, God overthrows nations for this very act:

Lev 18:19 “You shall not approach a woman to uncover her nakedness while she is in her menstrual uncleanness.
…
Lev 18:24 ‘Do not defile yourselves with any of these things; for by all these the nations are defiled, which I am casting out before you.
Lev 18:25 For the land is defiled; therefore I visit the punishment of its iniquity upon it, and the land vomits out its inhabitants.

Eze 18:5 But if a man is just And does what is lawful and right;
Eze 18:6 if he does not eat upon the mountains or lift up his eyes to the idols of the house of Israel, does not defile his neighbor’s wife or approach a woman in her time of menstrual impurity,
…
Eze 18:9 If he has walked in My statutes— And kept My judgments faithfully— He is just; He shall surely live!” Says the Lord GOD.

Eze 22:9 There are men in you who slander to shed blood, and people in you who eat on the mountains; they commit lewdness in your midst.
Eze 22:10 In you men uncover their fathers’ nakedness; in you they violate women who are unclean in their menstrual impurity.
Eze 22:11 One commits abomination with his neighbor’s wife; another lewdly defiles his daughter-in-law; another in you violates his sister, his father’s daughter.
…
Eze 22:15 I will scatter you among the nations and disperse you through the countries, and I will consume your uncleanness out of you.

In God’s law, having sex with menstruating women was a capital offense, one which led to the overthrow of nations, even Gentile nations. Prostitution, polygamy, promiscuity… not so much. While it may be shocking to modern Americans, plenty of activities were not criminalized in ancient Israel. And unlike having sex with menstruating women, God shows no strong disdain for these acts:

Women becoming prostitutes (there was a prohibition against cult prostitutes and against the daughters of priests becoming prostitutes).
Polygamy.
Premarital sex.
General promiscuity.
Divorce.

At best, these activities were scorned upon but tolerated (they were deviations from the ideal). At worst, these activities were not considered wrong (although in the case of prostitution there are plenty of New Testament texts claiming it is a sin). Jesus claims divorce was allowed for reasons of practicality. It could be the case that these other activities were also so widespread as to not allow general criminalization. But this still becomes a question of scale: homosexuality is wrong and a capital offense. Are there circumstances which God decriminalizes a large segment of homosexual acts on the grounds of practicality?

An intellectually honest Christian must understand that, by level of importance, God is willing to overthrow Israel for sex with menstruating women while similar attention is not shown to other activities. Equating adultery to polygamy is not a Biblical position. Although they might be technically equal, per Jesus’ understanding of the marriage bond, they were not the social or criminal equivalent (they have not even been consistently judged as equal in God’s economy). The care and attention shown each activity (activities that have always been widespread in human culture) is just not even on the same scale of the criminalized activities.

In the case of premarital sex, Paul’s advice is to marry:

1Co 7:2 But because of the temptation to sexual immorality, each man should have his own wife and each woman her own husband.
…
1Co 7:8 To the unmarried and the widows I say that it is good for them to remain single as I am.
1Co 7:9 But if they cannot exercise self-control, they should marry. For it is better to marry than to burn with passion.

This is in keeping with Old Testament laws on pre-marital sex, in which the “punishment” was that the two individuals should marry (but this could be overridden by the father).

Commonly misunderstood verses

Mat 5:28 But I say to you that everyone who looks at a woman with lustful intent has already committed adultery with her in his heart.

In this verse, Jesus is warning his audience (primarily married men) that if they look on married women with lust, they commit adultery in their heart. This is not a verse directed to or about unmarried men and women. The Greek word for woman is the same as for a married woman. Adultery cannot occur between people who do not have a sexual bond already formed. In this sense, Jesus affirmed what he is quoting:

Mat 5:27 “You have heard that it was said, ‘You shall not commit adultery.’

Not only should people not have sex with married women, they should not even lust after a married woman. Jason Staples expounds on this verse:

So it is clear that the grammar is reflecting purpose: “anyone who looks at a woman in order to covet her.” (“Covet” is preferable here in part because “covet” better reflects the intentionality reflected in the passage.) This is a critically important point; Jesus is not suggesting that any sexual thought or inclination towards a woman is sinful. Nor is he suggesting that such thoughts or attractions being triggered by a look are sinful. The look is not the problem (nor is the presence of a beautiful woman, which some of that day tended to blame as the real problem); no, these are assumed. What is remarkable (given the popular misinterpretation) is that Jesus likewise assumes the presence of sexual desire in the man as a given, and that sexual desire isn’t seen as the problem. Instead, Jesus addresses the matter of intent, of volition, the purpose of the look. The issue is not the appetite itself but how a man directs this natural appetite and inclination. (I’m reminded here of the old saying: If you’re a young man on a beach and a beautiful woman in a bikini walks past and you don’t feel any sort of excitement or attraction, it’s not because you’re spiritual, it’s because you’re dead.)

So Jesus’ statement was one which forbids man from looking upon a married woman with intent to covet. This is not about unmarried teenagers finding young, unmarried women attractive.

1Co 6:9 Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived: neither the sexually immoral, nor idolaters, nor adulterers, nor men who practice homosexuality,
1Co 6:10 nor thieves, nor the greedy, nor drunkards, nor revilers, nor swindlers will inherit the kingdom of God.

Often in the New Testament, sexual immorality is equated by modern Christians with modern notions of sexual immorality. Some equating is correct. Jesus considered sex as the marital act and bond, thus discounting the legitimacy of promiscuity, polygamy and prostitution. But Israel was still Israel, and inherited ancient Israelite notions of sexual immorality. Likely this phrase covers “having sex with menstruating women” or even “prostitution” (considering Paul’s treatment of prostitution elsewhere) or even the situation covered in 1 Corinthians in which a son is having sex with his mother-in-law (1Co 5:1). This would be a form of incest. Paul’s term “sexual immorality” might not cover normal or infrequent sexual promiscuity, and if it does, it is probably not on the same level as the sins that Paul describes.

Some further readings by an anti-Christian and anti-Jewish author:
Jews, Adultery and Prostitution in the Jewish Bible
Polygamy in the Jewish Bible