Augustine viewed the Original Sin, not as eating from the Tree of Knowledge, but as sex between Adam and Eve:
For it was not fit that His creature should blush at the work of his Creator; but by a just punishment the disobedience of the members was the retribution to the disobedience of the first man, for which disobedience they blushed when they covered with fig-leaves those shameful parts which previously were not shameful.
Although, if those members by which sin was committed were to be covered after the sin, men ought not indeed to have been clothed in tunics, but to have covered their hand and mouth, because they sinned by taking and eating. What, then, is the meaning, when the prohibited food was taken, and the transgression of the precept had been committed, of the look turned towards those members? What unknown novelty is felt there, and compels itself to be noticed? And this is signified by the opening of the eyes… As, therefore, they were so suddenly ashamed of their nakedness, which they were daily in the habit of looking upon and were not confused, that they could now no longer bear those members naked, but immediately took care to cover them; did not they–he in the open, she in the hidden impulse–perceive those members to be disobedient to the choice of their will, which certainly they ought to have ruled like the rest by their voluntary command? And this they deservedly suffered, because they themselves also were not obedient to their Lord. Therefore they blushed that they in such wise had not manifested service to their Creator, that they should deserve to lose dominion over those members by which children were to be procreated. [Letters of the Pelagians 1.31-32]
This view that sex was an evil was prevalent in Augustine’s time. Plotinus, a neo-Platonist that Augustine praises in his Confessions, taught that only through disdain for fleshly desire could one reach the ultimate state of mankind. Augustine, likewise, had served as a “Hearer” for the Manicheans for about nine years, and they also taught that the original sin was carnal knowledge. Augustine was definitely influenced by the Platonism of his time.
reality is not optional 
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I am currently writing a dissertation proposal to write on a topic which includes this idea. :-)
Isn’t Augustine actually saying that “the members by which sin was committed” are the “hand and mount, because they sinned by taking and eating”? He then asks why immediately after eating “the prohibited food” they suddenly decided to cover their genitals? His answer is that once they had sinned they became subject to involuntary sexual arousal and this embarrassed them.
“Although, if those members by which sin was committed were to be covered after the sin, men ought not indeed to have been clothed in tunics, but to have covered their hand and mouth, because they sinned by taking and eating.”
Augustine is posing a hypothetical. If the original sin was taking fruit and eating, then the hand and mouth should be covered. This is an argument against the literal reading.
OK, I see now how you are parsing that. But I am still not sure that is what he is saying. A little further on he offers the Pelangians four hypothetical options for how Adam and Eve would have performed the sex act before the Fall:
“But, while maintaining, ye Pelagians, the honourableness and fruitfulness of marriage, determine, if nobody had sinned, what you would wish to consider the life of those people in Paradise, and choose one of these four things. For beyond a doubt, either as often as ever they pleased they would have had intercourse; or they would bridle lust when intercourse was not necessary; or lust would arise at the summons of will, just at the time when chaste prudence would have perceived beforehand that intercourse was necessary; or, with no lust existing at all, as every other member served for its own work, so for its own work the organs of generation also would obey the commands of those that willed, without any difficulty. Of these four suppositions, choose which you please; but I think you will reject the two former, in which lust is either obeyed or resisted. For the first one would not be in accordance with so great a virtue, and the second not in harmony with so great a happiness. For be the idea far from us, that the glory of so great a blessedness as that should either be most basely enslaved by always following a preceding lust, or, by resisting it, should not enjoy the most abounding peace. Away, I say, with the thought that that mind should either be gratified by consenting to satisfy the concupiscence of the flesh, arising not opportunely for the sake of procreation, but with unregulated excitement, or that that quiet should find it necessary to restrain it by refusing.”
He simply assumes they would copulate. His only question is whether they would have experienced lust while doing so and how, if they didn’t, how Adam got an errection. He finds the idea that they were buffeted by powerful biological urges while yet in their sinless state unacceptable.