Who else could they have married!
Other females of the tribe who were evolving along with Adam and his family.
God forbidding incest coupled with Cain's marriage (as well as Seth's) is one of the best pieces of evidence for evolution in the Bible.
There is also no justification for "another tribe" in Genesis. God said that He created Adam and Eve, that they bore two sons, who are named, Cain and Abel, "other sons and daughters," and then Seth, as a replacement for Abel. The most straight forward reading can easily be reconciled as the best reading. Unfortunately, Hugh Ross does not seem to rely on the straight forward readings, and has "compromised" with naturalistic science to disallow the clear words of Scripture.
I find it interesting that someone who disparages compromise with naturalism will not answer a simple yes or no question: does the earth move. I also find it interesting that someone who disparages compromise can accept a morality that shifts at the whim of authority. I thought morals were absolute.