Where did Cain get his wife?

Where did Cain get his wife?

Many have argued that Cain simply married one of his sisters. I believe that this answer is a biblical impossibility. 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 supposedly 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 Christians 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 to himself.

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)?

Furthermore, Christians 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 the biblical god gives for His commandment.

The argument is often made that incest could not be inherently against god’s moral law if He blessed the marriage of Abraham and Sarah, who were half-siblings. This argument can be refuted quite simply. Does the god in the bible only bless people and their actions if they are sinless? Since none are considered sinless, the answer is obvious.