Early on I mentioned that if something is a sin today then it had to be a sin for The Adam and if something was a sin for The Adam then it must be a sin today. God does not change and God’s morals can not change. So who did Seth marry? The scripture does plainly teach that The Adam did have sons and daughters born to him after Seth, Genesis 5:4. However incest, in this case a brother taking a sister to wife, was a sin then just like it is now. Where did Cain find a wife and who populated the city he built?
The simple answer is that there were other people alive when The Adam was placed in the garden. I have been referring to this Adam with The to denote which Adam I am referring. I believe that Adam and Eve in Genesis chapter one are a previous creation of Adamites. I would suggest, and this is my guess, that God made several couples in Genesis one. Remember the word Adam is a description and not really a name. Although the scripture does refer to him with that word it describes him as well. It was this chapter one creation that provided husbands and wives for The Adam’s children.
Genesis chapter two then refers to Adam made after God rested as “The Adam” in the Hebrew. It was this Adam that is referred to as a son of God in Luke 3:38. Genesis chapter one and Genesis chapter two Adamites were all of the same type as the scripture uses the same descriptive term for all of them. They are the same people who have continued down through the ages eventually becoming Israelites. From the time of Jacob Israel to the present the scripture is mainly concerned about these same people
As far as timing goes we do know that the Genesis chapter two Adam was created almost six thousand years ago. Chapter one’s Adam could be considerably older. I have had several people tell me that I was wrong when I suggested that the six days of creation could possibly be one thousand years per day because of Peters statement in 2 Peter 3:8. “one day is with the Lord as a thousand years”. What is then ignored is that after The Adam sinned he was told that he would die that same day. However the day of his death was some nine hundred plus years later. That does fall into the 2 Peter 3:8 description. So I think it certainly is a possibility that we should at the least keep in mind.
So I would just leave this with a reminder that God was very careful to prohibit marriage with any people who were related to Israel through incest. I believe that God carefully preserved the seed line of The Adam through the birth of Christ as is demonstrated by the listings of the lineage in Matthew and Luke. I also believe that contrary to many God still has a plan for his true physical descendants of Israel as is clearly shown in both testaments. I suspect that this aspect will be looked at sometime in the future.