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, but that one I do not believe that Yahweh, the God of Abraham, Isaac and Jacob, would permit!
Incest is not an Option
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. The very nature of Yahweh is summed up in 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 (Yahshua) is the same yesterday, today, and forever."
This tells me that if YHVH (God) does not change then his morals can not change either. Therefore something that is sin today was sin in the days of Adam and Eve and if something was a sin then it is still sin today. Just read Leviticus. YHVH, through Moses, wrote down His Law, His moral Law, that had always been in existance. Not so you say? Many believe that there were no food laws before they were given in Leviticus. 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 be a marriage candidate and who can not for man. I believe that Adam knew those Laws. There are more articles on the subject of the Law at Oracles of Yah.
I don't know if people really think out the implications of what they believe having boys marry their sisters. This behaviour is only permitted in the writings of some who are anti-Christ. What I am going to suggest to you makes more sense to me than YHVH beginning civilization with no other way to go than commit incest (sin). Remember our God does not change. Incest and a man marrying his sister is a sin today and it was a sin in Adam's day.
We are called to use the mind that Yahweh gave us. I believe that the scripture teaches that there were other people alive and available for Adam and his boys to marry. 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 may have desired another wife or for sure needed wives for his boys. The idea that Adam married a second woman is merely speculative, but we know his boys had to marry in order to continue Adam's lineage. Any daughters that Adam might have had needed husbands as well. If what I am suggesting to you is not true 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. For an article on who one should not marry see Scriptural Marriage Possibilities. By the way, according to the book of Jasher, Abraham married his dead brother's daughter.
Feel free to send your helpful comments.