Who did Cain marry?

The identity of Cain’s wife has long troubled Christians. David Instone-Brewer proposes a new way of thinking about the problem 

It’s a question often accompanied by a smug smile: “So who did Cain marry?” If the reply is: “His sister,” the usual response is: “That’s immoral!” And if the answer is: “Not everything in Genesis should be taken literally”, the follow-up question is: “So how do we know which bits of the Bible are true?” 

Here’s another possible answer. It fits the literal Bible text better than Cain marrying his sister and also fits with all current scientific findings.