Why were Adam and Eve’s children allowed to marry?

Even though Adam and Eve sinned and became imperfect, their children were still very near to physical perfection that the offspring of brother and sister unions or other family unions did not suffer adverse effects as children born from such unions would today. In fact, people in ancient times lived in good health for hundreds of years. See Genesis 5:3-32. 

It is important to remember that no laws regarding marriage between close relatives existed. Even during the time of Noah there was no law preventing such unions. After the Flood (1656 years after the creation of Adam) only Noah, his wife and their three sons and their wives were left to inhabit the earth. God knew he had to repopulate the world so he temporarily allowed the marriage of close relatives. Also, man lived to be hundreds of years old. This was because man’s genetic structure was much more perfect than ours so as to be able to handle family members having close relations. 

More than 400 years later (some 2,000 years after Adam’s fall) God’s faithful servant Abraham married his half sister Sarah, and God did not disapprove (Genesis 20:12) as Abraham lived in a society where marrying half sisters was accepted as normal. It would be approximately another 450 years before God provided the nation of Israel with a body of laws that forbade such unions on penalty of death. See Leviticus 18:8- 18. By that time imperfection had apparently developed to such an extent that it was no longer safe for close relatives to marry as they had been doing in the past as allowed by God. 

Did God condemn the marriage of close relatives for the 2500 years prior to his giving Moses the Law? No, God allowed it in order to populate the earth. However, as man degenerated and his lifespan became shorter and shorter it was necessary to put strict laws in place for both health and moral reasons.