>>18143895 (OP)>god created adam and eve to give birth to humanity, but incest is forbiddenAugustine’s City of God, Book 15, Chapter 16 of Marriage Between Blood-Relations, in Regard to Which the Present Law Could Not Bind the Men of the Earliest Ages.
https://www.ccel.org/ccel/schaff/npnf102.iv.XV.16.html<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. <But there was then no material for effecting this, since there were no human beings but the brothers and sisters born of those two first parents. Therefore, when an abundant population made it possible, men ought to choose for wives women who were not already their sisters; for not only would there then be no necessity for marrying sisters, but, were it done, it would be most abominable. For if the grandchildren of the first pair, being now able to choose their cousins for wives, married their sisters, then it would no longer be only two but three relationships that were held by one man, while each of these relationships ought to have been held by a separate individual, so as to bind together by family affection a larger number. <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. For custom has very great power either to attract or to shock human feeling. And in this matter, while it restrains concupiscence within due bounds, the man who neglects and disobeys it is justly branded as abominable. For if it is iniquitous to plough beyond our own boundaries through the greed of gain, is it not much more iniquitous to transgress the recognized boundaries of morals through sexual lust?<and in marrying a cousin seemed almost to marry a sister,—for cousins are so closely related that they are called brothers and sisters,820 and are almost really so. But the ancient fathers, fearing that near relationship might gradually in the course of generations diverge, and become distant relationship, or cease to be relationship at all, religiously endeavored to limit it by the bond of marriage before it became distant, and thus, as it were, to call it back when it was escaping them. And on this account, even when the world was full of people, though they did not choose wives from among their sisters or half-sisters, yet they preferred them to be of the same stock as themselves. <The sexual intercourse of man and woman, then, is in the case of mortals a kind of seed-bed of the city; but while the earthly city needs for its population only generation, the heavenly needs also regeneration to rid it of the taint of generation. <But it does inform us that even these earliest of mankind sacrificed to God, as appeared also in the case of the two first brothers; Noah, too, is said to have offered sacrifices to God when he had come forth from the ark after the deluge. And concerning this subject we have already said in the foregoing books that the devils arrogate to themselves divinity, and require sacrifice that they may be esteemed gods, and delight in these honors on no other account than this, because they know that true sacrifice is due to the true God.