I found an article on the internet that was speculating that Adam may have had other wives from his progeny. The article also talked about who Cain, Able and Seth might have had available for marriage. The article's answer was that Adam could have married one of his daughters and that his boys married their sisters or mother's granddaughters. Sorry, I do not believe that Yah, the God of Abraham, Isaac and Jacob, would permit that!
We must always go back to the Scriptures for verification of any theory we might come up with to answer an apparent problem in the Bible. Yah's very nature is his morals. We know from two scriptures that He can not change.
"For I am YHVH, I do not change; Therefore you are not consumed, O sons of Jacob."
"Jesus Christ is the same yesterday, today, and forever."
This tells me that if Yahweh (God) does not change then his morals can not change either. Therefore incest was wrong in the days of Adam and Eve just as it is today. Just read Leviticus. Yahweh, through Moses, wrote down His Law, His moral Law, that had always been in existance. Not so you say? In Leviticus we have the food laws, but Noah knew the food laws before the flood. When YHVH told Noah to take seven clean and 2 unclean animals of each kind he knew what to do. Also Abraham was credited by YHVH as one who "obeyed My voice and kept My charge, My commandments, My statutes, and My laws", Genesis 26:5. That was a long time before Moses. Leviticus tells us who can not be a marriage candidate for man.
So where does that leave us. That puts us in between, perhaps, a rock and a hard place? Not really. This idea may not be the only answer, however I think it makes more sense than YHVH beginning civilization with no other way to go than commit the sin of incest.
We are called to use the mind that Yahweh gave us and my mind says that there were other people alive and available for Adam and his boys to marry. Some say that the creation accounts of Genesis 1 and Genesis 2 are separate creations and so the Genesis 1 created people (Adam the gatherer and the other races) were already around when Adam (the farmer, gardener) in Genesis 2 needed wives for his boys. We know his boys had to marry in order to continue Adam's lineage. Adam's daughters needed husbands as well. I lean toward the idea that Genesis 1 and 2 are separate creation accounts. So then his children had mates from the descendants of Adam the gatherer. If Genesis 1 and 2 are not separate accounts then the Bible is very much silent concerning other people being created. Silence, however should not be used to prove that something never occurred. The Bible is silent about many things that we know did happen.
Well, that is my answer to the problem. Do you have any better ideas? Let me know.