When does gods' law become the basis for moral behavior?

We are often told that the bible is the word of god and that morality comes from gods laws. But that opens up questions.

Why was Cain guilty of murder although the Mosaic Commandment had not been given, and why were the Sodomites guilty of sexual sin before the Mosaic commands against homosexual behavior? If no one knew it was against gods will, why would we say it was bad? Now, one can argue that these rules are imprinted on our consciousness. But if that is true, there would be no reason for god to give any rules or laws. We would simply know what they are. But he does give them. Therefore, either we do not simply know the rules or they are not rules before god sets them into law.

Where Cain gets his wife or a woman to bear him children is a question that many debate. There are only three options. He had sex with his mother. He married one of his sisters or cousins. There were other families which brings into question the Genesis story.

Most have argued that Cain simply married one of his sisters. I believe that this answer provides a paradox. . Those who hold to this answer concede that God gave laws forbidding sexual acts between close relatives (Leviticus 18; Deuteronomy 27) but they argue that God did not give these laws until the time of Moses, and so early humans, including Cain, were excluded from these laws. However, this argument fails for the reason that God’s moral law is unchanging. If the moral law of God has no meaning until it is given, then Cain also did no wrong in murdering his brother Abel, and yet God holds him responsible for it. For the ‘sister argument’ (if I may call it that) to work, incest must be considered ceremonial or civil law, but it is clear from the context of Leviticus 18 and Deuteronomy 27 that we are to consider incest to be moral law. If it is indeed moral law, then it is impossible for God to have commanded mankind to multiply and fill the earth by means of incest, for God would have been commanding disobedience unto Himself.

Furthermore, we must consider the reason that God gives for forbidding incest, or as the Bible expresses it, uncovering the nakedness of a relative: ‘for their nakedness is your own’ (Lev. 18:10). Those who argue that Cain married one of his sisters claim that the law against incest was only given at a later time in order to protect people from biological defects, but that is not the reason that God gives for His commandment.

If gods laws are eternal and unchanging, then Cain could not have committed incest. Therefore, there must be another source for Cain's wife besides a family member. But there is no other source. Therefore, the Genesis account cannot be accurate.