Update: Apparently it's well-understood by anyone who goes to bible school that Cain & Abel did marry their cousins or sisters. (I interviewed many a staunch christian on this matter to get some answers)
As for the matter of incest - as they were the first humans, their bodies were supposedly quite pure and not chock-full of diseases like we are, hence no worries about double recessive genes causing disfigured children with terminal disorders.
While re-reading Genesis 4, it hit me loud & clear:
Where did Cain & Abel get their wives from?
If Adam & Eve were the first humans on earth, and Cain & Abel were the first and only offspring of the first humans on earth... Where did the other humans come from?
Namely, the humans that Cain & Abel were taking to be their spouses?
This doesn't make sense.
My best guess is Adam & Eve had more offspring - female children this time. And married them off to their two sons.
I know. INCEST GALORE.
How else to explain this then?
If this is true, then, I'm sorry to say this, but it means every single one of us is an inbred.