Where did Adam and Eve's grandchildren come from?

You see, the fundy's can't accept the fact that the story of Adam and Eve is just a myth. So they indeed do have a little problem here. So they have to come up with all sorts of contorted answers to solve this problem - all of which are just as irrational as accepting the Adam and Eve story as literal fact. Their explanations range from God having to create extra people that he never told us about, to making excuses for at that time because it was biologically necessary - the gene pool was not sufficiently degraded. God preserved it or some such nonsense.