The unsaved skeptic understands the basic premise of what happened, but makes a couple mistakes in their logic. They understand the Adam and Eve were the first two human beings created, and that the Bible mentions their sons Cain and Abel. Cain murdered his brother Abel and went into exile.
The first assumption they make is that Cain was not married at the time he murdered his brother. The Biblical narrative does not say anything about his marital status before he killed Abel, he just is sent into exile. He could have been married, so the problem of him finding a wife once in exile probably would not have been an issue. We also don’t know how far his other brothers and sisters would have scattered from outside of Eden, so that even if he was single when he went into exile, he could have found a wife.
The second and most pressing assumption they make is that the rules on incest that we presently have would have been present soon after creation. It is a common fallacy that if something is true today, then it must have been true in the past. This ignores a few basic facts of science and scripture.
1. In the first generations after creation the human genome would have had minimal corruption from mutations and other cellular damage. There would have not been the dangers associated with marriage among siblings, cousins, aunts, and uncles that there presently is. Due to modern problems it is not safe for humans to marry closer than a third cousin, the child of one’s parent’s cousin. Closer to creation, the few mistakes there would have been in the human genome, the safer it would have been for siblings, cousins, etc. to marry.
2. God prohibited homosexuality and sexual relations outside of marriage, but the rules about siblings intermarrying was only prohibited by the time of Moses. Noah’s grandkids would have had to intermarry after the flood since the human race was only Noah, his wife, Ham and his wife, Shem and his wife, Japeth and his wife. Abraham was married to Sarah, who was his half sister. Moses’ father, Amram was married to Jochabed, who was his aunt. It was not wrong for close relatives to marry.
The logical conclusion of all of this is that Cain’s wife was either his sister, a niece, or grand niece of some sort. It is not that hard to figure out…just logic.