How Did Cain Get His Wife?
The answer is actually quite simple - he married his sister who simply happens not to be mentioned in the Bible. But, isn't that immoral to marry your sister? Yes, it is today. That is because we know that if siblings marry each other, they are very liable to have deformed children, and that, of course, would not be fair to the children and therefore immoral. However, these deformities are the result of genetic defects in our DNA. From a Biblical world view, we conclude that these defects are the result of sin and not a part of God's original creation. These defects have come about as a result of mutations which have been accumulating in our gene pools since the fall of Adam and Eve. The DNA of Cain and his wife who were the generation just after Adam and Eve was assuredly much less likely to cause deformities than is our DNA today, even when we don't marry close relatives.
Did Adam and Eve have any daughters? Most assuradely! Genesis 5:4 states: "And the days of Adam after he had begotten Seth were eight hundred years: and he begat sons and daughters."