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.