Good question. In Genesis 5:4, Scripture tells us that Adam and Eve had "other sons and daughters." It also tells us that Adam lived several hundred years (Gen. 5:5). This would mean that his children married each other.
So does this mean incest is allowed by God? No. Incest (i.e. sex between family members) is prohibited by God in Lev. 18:6-18, 20:11-12 and 20:19-21. But in such an early stage of human development as in the time of Adam and Eve, it was permitted as the genetic line was still pure. God allowed incest for a specific time and for a specific purpose - namely so that the human species could reproduce and "fill the earth" (Gen. 1:28).
When Leviticus was written at a much later date, the genetic line had become more unstable and therefore God commanded his people not to marry their relatives. Biologically, because relatives already share some genetic material, there is a greater risk that incest will result in a child who has a rare genetic disorder carried as arecessive trait. Recessive traits may cause no symptoms at all in those that carry the genes, but whencombined can result in seriously affected offspring. The closer the relationship of people in an incestuous relationship, the greater the risk that they both carry the same recessive genes.Even today, it has been shown that incest increases the chances of having children born with genetic diseases.
If you want to find out more, you can read this article which answers the question, "Is Incest Okay?" or this one, which gets into the real specifics of reproduction during the time of Adam & Eve.
[Answered by Pastor HM]
