If we believe the story of creation, then the world was recreated through incest in the strict interpretation of the word. If all mankind stemmed from Adam and Eve then there was incest by necessity which is demonstrated throughout the old testament. what created the need for laws against it.
Leviticus 18:1 - 30
NASB - 1 Then the LORD spoke to Moses, saying. 2 Speak to the sons of Israel and say to them, 'I am the LORD your God.
For follow-up discussion and general commentary on the topic. Comments are sorted chronologically.
Glory to The Creator for giving us the creation and the ability to also create/think. Think of time and place. In a small community with less people, there's a greater chance you will procreate with a blood relative. Small towns and villages are easier to maintain. New blood doesn't come in much. Marriage to a family member is just procreation. Consider Adam/Eve,
Compare a village to a large town or a city. There's more races, property, money and opportunity. If you procreate and marry there, both parties gain. The marriage is closer to a long term business deal. Consider Isaac getting married to a distant family member. It was more a business deal and he managed to gain from it in a major way. Like the rich marrying the rich to get richer.