Question: "Why did God allow incest in the Bible?"
Answer: There are numerous examples of incest in the Bible. The most commonly thought-of examples are the sons/daughters of Adam and Eve (Genesis 4), Abraham marrying his half-sister Sarah (Genesis 20:12), Lot and his daughters (Genesis 19), Moses’ father Amram who married his aunt Jochebed (Exodus 6:20), and David’s son Amnon with his half-sister Tamar (2 Samuel 13). It is important to note, however, that in two of the above instances (Tamar and Lot) one of the parties involved was an unwilling participant in the incest.
It is important to distinguish between incestuous relationships prior to God commanding against them (Leviticus 18:6-18) and incest that occurred after God’s commands had been revealed. Until God commanded against it, it was not incest. It was just marrying a close relative. It is undeniable that God allowed incest in the early centuries of humanity. If Adam and Eve were indeed the only two human beings God created, their sons and daughters would have had no other choice but to marry and reproduce with their siblings and close relatives. The second generation would have had to marry their cousins, just as after the flood the grandchildren of Noah would have had to intermarry amongst their cousins. The reason incest is so strongly discouraged in the world today is the understanding that reproduction between closely related individuals has a much higher risk of causing genetic abnormalities. In the early days of humanity, though, this was not a risk due to the fact that the human genetic code was relatively free of defects.
It seems, then, that by the time of Moses, the human genetic code had become polluted enough that close intermarriage was no longer safe. So, God commanded against sexual relations with siblings, half-siblings, parents, and aunts/uncles (Genesis 2:24 seems to indicate that marriage and sexual relations between parents and children were never allowed by God). It was not until many centuries later that humanity discovered the genetic reason that incest is unsafe and unwise. While the idea of incest is disgusting and abhorrent to us today, as it should be, we have to remember why it is sinful, that is, the genetic problems. Since this was not an issue in the early centuries of humanity, what occurred between Adam and Eve’s children, Abraham and Sarah, and Amram and Jochebed, should not be viewed as incest. Again, the key point is that sexual relations between close relatives must be viewed differently pre-Law and post-Law. It did not become “incest” until God commanded against it.