Where did Cain get his wife?
Well, he married his sister! How can we know that for sure?
The confusion comes because people think Adam and Eve only had Cain and Abel: so who did they end up marrying?
Later on, it says that Adam and Eve had sons and daughters. (Gen 5:4) The reason no other names are really listed other than Seth was that these were important to the story line, and also Seth is in the lineage of Christ so this is very important also.
Today we think that is completely gross and disgusting, but the reason is, that it is not accepted today by man or by God because it is totally unsafe!
After the flood, dangerous radiation effected the atmosphere that automatically effected the human genetic code! It began to degrade and we also see a massive decline in life expectancy.
If brothers married sisters today, the children would be deformed, and have some serious mental issues because brothers and sisters would have the same mistakes in their DNA and if you married you would multiply those genetic mistakes!
Back in Adam and Eve’s days, the genetic code was near perfection. The further out in time, you see a genetic break-down to the point that in Leviticus God said that incest was no longer allowed. It was too dangerous.