A natural question arises concerning Adam and Eve’s children. Who was Cain’s wife? If there were no other humans besides those who came from Adam and Eve, then by definition wouldn’t Adam and Eve’s children be committing incest? Cain would have no option except to marry his sister, which would be incest!
If Cain married his sister, that would seem to be wrong because of Genesis 2:24’s definition of a “one flesh” relationship? Cain is not the only one who might be guilty of incest by that definition. Abraham married his half-sister (cf. Gen 20:12). At the very least, this would seem to be poor planning by the Creator; at the worst, it would be a plan where the Creator causes His creatures to sin.
Although there have been a variety of answers to question of who Cain’s wife was and why apparent incest was allowed, one solution holds promise above others. If we consider what incest actually is, then we can say that incest did not occur until later in earth’s history.
Incest can be simply defined as a union between those who are related. Or, to put it another way, a union between two individuals who share certain degree of “sameness.” If two people share a certain degree of sameness, then they cannot create a new “one flesh” relationship (cf. Gen 2:24) since they already share sameness. However, at the beginning of the human race, it is possible that even those of the same family still had a significant degree of difference (i.e., the family of Adam and Eve). Therefore, brother and sister could be married because they were still significantly different from one another and did not share a significant amount of “sameness.”
This line of reasoning seems to be confirmed if we compare the early history of Scripture with what we know about genetics today. When Adam and Eve were created by God, they contained the perfect, undamaged genetic information for the entire human race. However, due to sin genetic mutations entered the human genome and slowly developed into pronounced ways over time.
After the flood, the human genome was completely contained in Noah and his family. As humanity spread out and segregated into various groupings (post Babel), the genetic information became more and more diluted through mutations and isolation. By the time Israel became a nation, enough time and genetic mutations had occurred that there was significant “sameness” that existed between family units. Therefore, in order to preserve the uniqueness of new “one flesh” relationships, as well as to preserve humanity from the deleterious genetic effects of intermarrying into the same genetic mutation patterns, God made clear that marrying close relatives was now forbidden. This would explain why marriage between family members in the pre-Israel time period was not taboo, but by the time God gives the Law to Moses (ca. 1446 BC) there is a significant amount of sameness that exists in people groups, and thus incest is a real possibility as described in Leviticus 18.