One of the reasons many Christians cannot answer the question about Cain’s wife is that they tend to look at today’s world and the problems that would be associated with close relations marrying, and they do not look at the clear historical record God has given to us.
They try to interpret Genesis from our present situation rather than understand the true biblical history of the world and the changes that have occurred because of sin. Because they are not building their worldview on Scripture but taking a secular way of thinking to the Bible, they are blinded to the simple answers.
Genesis is the record of the God who was there as history happened. It is the Word of One who knows everything and who is a reliable Witness from the past. Thus, when we use Genesis as a basis for understanding history, we can make sense of evidence which would otherwise be a real mystery. You see, if evolution is true, science has an even bigger problem than Cain’s wife to explain—namely, how could man ever evolve by mutations (mistakes) in the first place, since that process would have made everyone’s children deformed? The mere fact that people can produce offspring that are not largely deformed is a testimony to creation, not evolution.