Answer # 33

The answer is found in Genesis 5:4-5: "And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters: and all the days that Adam lived were nine hundred and thirty years: and he died"

Notice that Adam begat daughters. By this time other children had been born to Adam and Eve. It is quite obvious that Cain married one of his sisters - Adam's daughter. There simply weren't any unrelated females for him to marry.