Thought from Today's Old Testament Passage:
Cain lay with his wife, and she became pregnant. (Gen. 4:17)
Immediately we want to know, where did Cain get his wife?… This question has been asked so many times that it no doubt deserves a place in the Biblical Hall of Famous Questions.
The most obvious answer must be that Adam and Eve had other children than those explicitly mentioned thus far, including daughters. Indeed, Genesis 5:4 plainly says as much, "[Adam] had other sons and daughters."
The point is that Cain must have married his sister. But to admit this is to raise a further difficulty. Was he guilty of incest if he married his own sister?
At least two things can be said in response to this reproach. First, if the human race was propagated from a single pair, as we believe the evidence indicates, such closely related marriages were unavoidable. The demand for some other way of getting the race started is an unfair expectation.
In the second place, the notion of incest must be probed more closely. At first the sin of incest was connected with sexual relationships between parents and children. Only afterwards was the notion of incest extended to sibling relationships….
The genetic reasons for forbidding incest were not always an issue. Close inbreeding in ancient times was without serious or any genetic damage. Today, the risk of genetic damage is extremely high. Since the genetic possibilities of Adam and Eve were very good, there were no biological reasons for restricting marriages to the degree that it became necessary to do so later. (Walter C. Kaiser, Jr., Hard Sayings of the Old Testament (Downer's Grove, IL: InterVarsity Press, 1988), pp. 38-39)