I have never given this much thought until now. This question has got me intrigued. I'll give a few of the explanations that I have found.
The classic idea is that Cain married his sister.
Genesis Rabbah states "Cain was a twin, for with him was born a girl; and Abel was one of
three, for with him came two girls."
And in the Talmud
Sanhedrin 58b "Come and hear! Why did not Adam marry his
daughter? So that Cain should marry his sister, as it is written, For I said, the world shall be built up by grace. But otherwise, she would have
been forbidden [to Cain] - Once however that it was permitted, it remained so."
As has been already been posted by kingofmd, the prohibition of marriage between brother and sister (see Lev. 18:9) was not in effect until the law
was given at the time of the exodus. After that time, the practice was considered abhorrent so no mention of her is given.
Which caused me to ask myself another question. Why are any of the descendants of Cain even mentioned. I have not found any real answer. The
closest thing I could find is again from
Genesis Rabbah which states "Naamah, daughter of
Lemech and sister to Tubalcain, was Noah's wife." As she is the last mentioned in the lineage of Cain, that is the best reason I can find for
that.
There is another theory that I remember hearing, but cannot find any references for it. I think this one is silly, but I'll throw it out anyway.
It is based on Gen. 3:16 "To the woman He said, 'I shall surely increase your sorrow and your pregnancy; in pain you shall bear children. And to
your husband will be your desire, and he will rule over you.'"
The idea is that without the pain of childbirth there is the possibility that she may have had an uncountable number of children. Like I said, it is
goofy and nonsensical.
Those are the classic explanations. I have a somewhat different take on it. I don't take these stories as a literal fact. I look at these as an
allegorical story of how things came to be. Gen 1 describes an evolutionary concept. First simple life, grasses and plants; then marine life; then
terrestrial life; then humans.
Gen 3 has been given a bad rap as "The fall of man". I don't see it like that. I see the expulsion from the garden as a good thing. In the
garden, they were running around naked and picking nuts and berries. Then they are kicked out and begin to wear clothes and begin agriculture. It is
a great step forward to civilized life. It is a graduation from a hunter-gatherer existence to a civilized agricultural existence.