If we take the Bible verbatim the answer is very simple.

The solution lies in the fact that Adam and Eve had more than two children. At Genesis 5:3 we read that Adam became father to another son named Seth and then, in the following verse, we read: “He became father to sons and daughters.” (Genesis 5:4) So Cain could have married one of his sisters, that weren't mentioned, or even one of his nieces.

This sounds like it violates some of the Biblical laws; however, though there are strict laws against sex between parents and children, it really doesn't say anything about sex between siblings, and there aren't any incest law against sex relations between uncles and nieces. And even so, those laws came later than Cain times, and we can safely say that Cain Did not violate any divine law.

I don't think that to go further than that is necessary or logic, and any kind of alien or other bizarre solution is to stretch the Biblical text to far.