Which I suppose is why Genesis glosses over the necessity of Cain, Seth, etc. in those first generations marrying sisters or first cousins.
That whole section of Genesis is problematic if we assume that Adam, Eve, Cain, and Abel are the only people in the whole world. In Chapter 4, after Cain kills Abel, Jehovah curses him to be a fugitive and a vagabond. Cain whines that "every one that findeth me shall slay me." (That's a weird phrasing by the translators of the KJV -- surely Cain doesn't expect to be slain multiple times. But I digress.) So Jehovah puts a mark on Cain so that nobody will kill him.
Who exactly is Cain afraid of at this point? It would seem that there were only three other people in the world, and he just killed one of them. The other two are his parents, who a.) would recognize him even without Jehovah's mark on him; and b.) he wouldn't be likely to meet again as he's been sent away from them to wander the earth. The rationalization I've heard most often is that Cain expects Adam and Eve to have another son who'll in turn have a whole bunch of descendants, and further expects that he'll live long enough to eventually encounter one of his great-great-grandnephews or whatever, who'll kill him because he's a fugitive and a vagabond.
But wait. Cain goes out into the Land of Nod, and the next thing you know, he's got a wife. I think there are three possibilities here:
1.) Adam and Eve had an unnamed daughter who was also exiled to the Land of Nod, and she became Cain's wife;
2.) There's a huge time gap between Genesis 4:16 and Genesis 4:17, during which Adam and Eve have a bunch more sons and daughters, and grandsons and granddaughters and so on, and they go on to populate Nod (and other nearby Lands), and one of these distant cousins becomes Cain's wife;
3.) There were already other people living in Nod and the other lands, people unrelated to Adam and Eve, possibly created by other gods than Jehovah, and Cain married one of them.
Option 1 is the flimsiest explanation, especially since Cain builds a city in Genesis 4:17 and names it after his son Enoch. It's unlikely that three people would really constitute a city.
Option 2 is, I believe, preferred by most literalists and inerrantists today, although I could be mistaken on that.
Option 3 is the only one that makes any sense to me, but I suppose the theological implications make it unpopular with your hardcore Bibliolators.
And then we come to Genesis 6, where Jehovah gets worried that the "sons of God" are marrying "the daughters of men," and especially the very weird verse 4, where the sons of God and the daughters of men give birth to giants or Nephilim or "mighty men of old". Or something. But it also implies that there are people (or very people-like beings who are interfertile with people) who are distinct from Adam's descendants.
