Cain Married His Sister - Why Is it Wrong Now?

St. Augustine and CMI are united on two things.

• Cain married his sister;
• marrying one's sister would be wrong now.

They are also disunited on why it was right then and wrong now, a common point on why it was right being sheer necessity:

• CMI says, it's the genes : as mutations accumulate, sharing a mutation becomes more dangerous.
• St. Augustine says, maximising the number of friendships requires to have a different father and father in law, if possible.

Here are the core explanations, which are different:

CMI: Who was Cain’s wife?
The Creation Answers Book (8th ed. 2019), Chapter 8
https://creation.com/who-was-cains-wifeMI

This explains why, when two people marry today, their children rarely show mutational defects. The point is that even though each parent carries hundreds of mistakes, and passes many on,12 the mistakes carried by each parent are not usually the same sets of mistakes. Because a husband and wife usually have parents with quite different genetic backgrounds, they will have significantly different sets of mistakes. So any defective gene inherited from one parent will normally be ‘covered up’ or ‘compensated for’ by the normal gene, carrying the normal instructions, passed on from the other parent.

St. Augustine : 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.newadvent.org/fathers/120115.htm

... For it is very reasonable and just that men, among whom concord is honorable and useful, should be bound together by various relationships; and 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. "Father" and "father-in-law" are the names of two relationships. When, therefore, a man has one person for his father, another for his father-in-law, friendship extends itself to a larger number. But Adam in his single person was obliged to hold both relations to his sons and daughters, for brothers and sisters were united in marriage. So too Eve his wife was both mother and mother-in-law to her children of both sexes; while, had there been two women, one the mother, the other the mother-in-law, the family affection would have had a wider field. Then the sister herself by becoming a wife sustained in her single person two relationships, which, had they been distributed among individuals, one being sister, and another being wife, the family tie would have embraced a greater number of persons. 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. For one man would in that case be both father, and father-in-law, and uncle to his own children (brother and sister now man and wife); and his wife would be mother, aunt, and mother-in-law to them; and they themselves would be not only brother and sister, and man and wife, but cousins also, being the children of brother and sister.

Two things should be retained from this discussion:

• St. Augustine (and in general all the Church Fathers) agreed because this is what the Bible text says.
• Catholics have a notion of natural law or equity, while Protestant specialists have a notion of God knowing things that those obeying Him do not know.

While that is also a fact, this was in fact obeyed by most pagans too, not to marry sisters, so, this means, the Church Father has a point in favour of Natural Law./HGL