system
A chat on subreddit /r/NoStupidQuestions
human
One major theory about the story of Adam and Eve is that it was intended to be the origin of the Jewish people, not of humanity as a whole. As such, the idea that there were women outside that Cain and Abel could marry doesn't seem as ridiculous.

The trick is that nowadays, we don't find the concept of a religion about a specific tribe to be as compelling, and more universal ones tend to be common.
gpt
No. The Bible says Abraham was the father of the Jews. In the narrative, "Jews" came into existence over a thousand years after Adam, who was intended to be the father of the human race. The earliest surviving Jewish commentaries on the story, such as Josephus's, agree that Cain, Abel, and Seth married their sisters.