Genesis Rabbah was completed around 400 C.E. Neusner quotes Genesis Rabbah 70:12, which is interpreting Genesis 29:11. Genesis 29:11 states that Jacob kissed Rachel and wept. The rabbis want to know why Jacob wept. Here is one answer:
Why did Jacob weep? Because he saw that men were whispering with one another, saying, “Has this one now come to create an innovation in sexual licentiousness among us? [That is something we cannot permit.]” For from the moment that the world had been smitten on account of the generation of the flood, the nations of the world had gone and fenced themselves away from fornication. That is in line with what people say: “People of the east are meticulous about sexual purity.”
The passage states that, after the flood, the nations of the world took a stance against fornication.
This reminds me of something Tim Keller once said when he was preaching against sex outside of marriage: “You know how some say we should come together and celebrate what all religions and cultures have in common? This happens to be one of those things. Most cultures in the world are against sex outside of marriage.” This isn’t an exact quote, and there are plenty of countries that are liberal on sex, especially in Europe. But there are also many that are quite traditional.
How does this rabbinic passage fit with the Scriptures? Pharaoh and Abimelech didn’t believe in adultery, for they were upset when they found out they almost slept with Abraham’s wife (Genesis 12; 20). I suppose the people of Sodom and Gomorrah weren’t all that chaste, since they tried to gang-rape Lot’s guests (Genesis 19). Leviticus 18 lists a bunch of bizarre sexual acts and says they were practiced by the Egyptians and the Canaanites. Maybe the nations were against fornication for a while and eventually gave in to their flesh. But, by and large, the Gentile nations were against adultery. That’s why the Israelites were especially bad when they were neighing after their neighbors’ wives (Jeremiah 5:8)!