Did Cain marry his sister?

The account in Genesis does not claim to list all the children of Adam and Eve  by name, but makes it clear that there were others: Genesis 4:17 mentions that Cain had a wife. Gen 4:25 mentions the birth of Seth, another son of Adam and Eve. 4:26 mentions Seth’s son, Enosh. Finally, 5:4 says, “Adam … had other sons and daughters.” Thus it can be safely assumed that Cain’s wife was a daughter of Adam and Eve as there was no other source.

At the beginning of the human race, marriage between close relations was necessary and unavoidable but was closed off once the necessity had passed.