City of God - Book 15, Section 16
Augustine addresses the incest problem implied by a literal interpretation of Genesis, which would have required Cain and Abel to marry their sisters. He discusses the thought processes that drive people to intermarry with their close relatives but render the closest of these relationships naturally abhorrent. He concludes that the circumstances of Adam and Eve’s first children were unique and what was evident to later generations would not have occurred to them. He also points out that the law of Moses which prohibited the marriage of siblings was not written until a much later era.
As far as I can follow it, Augustine's argument is that it is unnatural to seek to limit the size of one’s family, since a larger family is always better; and simple familial respect prohibits one from feeling sexual attraction for a close relative, someone with whom you grew up; but neither of these thought processes would have naturally occurred to people for whom the only available sexual partners were their own siblings.
Of marriage between blood-relations, in regard to which the present law could not bind the men of the earliest ages.
As, therefore, the human race, subsequently to the first marriage of the man who was made of dust, and his wife who was made out of his side, required the union of males and females in order that it might multiply, and as there were no human beings except those who had been born of these two, men took their sisters for wives,—an act which was as certainly dictated by necessity in these ancient days as afterwards it was condemned by the prohibitions of religion. For it is very reasonable and just that men, among whom concord is honourable and useful, should be bound together by various relationships; and that one man should not himself sustain many relationships, but that the various relationships should be distributed among several, and should thus serve to bind together the greatest number in the same social interests. […]
And we see that, since the human race has increased and multiplied, this is so strictly observed even among the profane worshippers of many and false gods, that though their laws perversely allow a brother to marry his sister,[182] yet custom, with a finer morality, prefers to forego this licence; and though it was quite allowable in the earliest ages of the human race to marry one's sister, it is now abhorred as a thing which no circumstances could justify. […] [T]here is in human nature I know not what natural and praiseworthy shamefacedness which restrains us from desiring that connection which, though for propagation, is yet lustful, and which even conjugal modesty blushes over, with any one to whom consanguinity bids us render respect[.]
Modern Creationists have contributed to Augustine’s argument by conjecturing that the first people did not have genetic defects like we do today, and thus could have intermarried with their close relatives without their offspring suffering from the sort of deformities and deficiencies that are common today among children born to closely related parents.