“You say the stories in the Bible are true history. So where did Cain get his wife?”
If the story is taken as legend or myth, of course, there is no problem. But if the story is taken as literal history, how could Cain have a wife if he and Abel were Adam and Eve’s only children? Of course, the Bible does say that they had other children (Gen 5:4); his wife could have been a sister. The incest taboo began as prohibiting sexual relations between parents and children and only later extended to relations between brother and sister. But rather than take this rather anthropological approach, I suggest it is better simply to acknowledge that the Bible makes no attempt to deal with this question and so neither will you. We must deal with what ancient texts say, not what they omit. Surely if there were anything important to be learned from how Cain got his wife, we would have been told. The point of the opening chapters of Genesis is to deal with the purposeful origin of human life as created by God. The richness of the implications of what we are told about human origins are scarcely exhausted by all our attempts to understand them.
— James Sire, from Why Good Arguments Often Fail, p. 199