I've heard dozens of justifications: God created Cain's wife specifically for him; there were people outside of Eden before Adam & Eve left-- from evolution or seperate creation; she was an angel; she was an animal or demon; she was Cain's sister; Cain was asexual, and reproduced by budding; Cain's supposed offspring were merely others who disobeyed God, not a result of procreation...
     But it doesn't matter.
     How can I say that? Simple. In historical discussion, one does not debate what cannot be proven by sources-- only the implications of those sources. Cain's wife existed. There is no writing about how she came to be, period. So it is irrelevant. It does not change the story.

     *** more to come ***