For Those of You Who Believe in Strict Creationism...

[The Mikado]

5:3  And Adam lived an hundred and thirty years, and begat a son in his own likeness, and after his image; and called his name Seth:   
5:4 And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters:

Cain married his sister.

[The Mikado]

5:3  And Adam lived an hundred and thirty years, and begat a son in his own likeness, and after his image; and called his name Seth:   
5:4 And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters:

Cain married his sister.

But the Bible very clearly forbids incest.  Thus, he would have had to have married a non-existant woman.

Just for argument's sake...Cain was a man who killed his own brother, and God didn't take a real light to fratricide.  Do you think the prohibition against incest would have kept Cain from going after one of his little sisters?  (Or his mother: Eve was still around, remember.)

[The Mikado]

The prohibition against incest didn't exist until Moses' times.  Before then, the genetic complications from incest wouldn't have been strong enough to warrant prohibition.  Recall that genetic disorders and the like were only a result of our original fall from grace (thanks a lot, Eve), so they could only have gotten progressively worse.

Thus, not only did Cain marry his sister, but it was perfectly moral and safe to do so.

Now what is distressing is that the writer of Genesis didn't see fit to mention Adam and Eve's female children, because apparently women aren't as important as men.  I wonder what Cain's sister thought about the prospect, or whether she had any choice in the matter.  My guess is that she did not.

That's one of the more desperate explanations I have heard (assuming is wasn't a joke).

First, your theory defeats itself because there never would have been enough genetic variation created to begin with in order to produce enough genetic variation for there to be enough genetic variation for there to be genetic complications from incest.

Also, Adam and Eve had their children after the fall.

Finally, Genesis was written at the same time as Exodus, so odds are that the people taking it down would have had the same thoughts in mind.  All except Genesis 1, which was written thousands of years later, for a different audience, during a different time, hence why the tone of Genesis 1 is so different from the latter sections and why it seems to kinda contradict Genesis 2.

But incest is still all over Genesis (Lot and his daughters and Judah and Tamar).  It doesn't mean the authors are condoning it, and, in fact, Cain, like Lot, is none-too-popular in the book.  I wouldn't at all be surprised if the authors were intentionally putting the shame of incest on him.