Who was Cain's wife, where did she come from, and were they related?

     One of the questions that always popped into my head after reading Genesis was, "Who did Cain marry?" After he murders Abel and God banishes him, "Cain went out from the Lord's presence and lived in the land of Nod, east of Eden.  Cain lay with his wife, and she became pregnant and gave birth to Enoch..." (Genesis 4:16-17).  Noting that this does not say he "found" his wife in the land of Nod, only that he "lay with" her there, it still vexed me for a while, If Adam and Eve were the first and only people on Earth then where did she come from?  Were there other people on Earth not related to Adam and Eve that the Bible does not mention?  There are way too many problems that scenario would cause to the Bible story, mostly in the idea of "in Adam's fall, we sinned all." If we are not all descended from Adam and Eve then the entire human race could not be condemned to death for his transgression and, therefore, would not need a savior...which would take away the overall message of the Bible.
     Okay, so Cain's wife had to have been of his own bloodline.  Besides, Genesis 5:4 does say that after Seth, which was the third son mentioned by name, that Adam lived 800 years "And he fathered sons and daughters."  One could look to the Book of Jubilees, which "purports to be a revelation given to Moses on Mt. Sinai by an angel of the Presence" for a more specific answer (Barry Smith, Introduction to the Book of Jubilees).  According to it, "Cain took Awan his sister to be his wife and she bare him Enoch" (iv 9).  So, why would this not have created deformed offspring, like you would see today if a close relative, especially a sister, bore a man's child?  Not to mention the Bible outlaws the union of such close relatives.  To address the latter issue first, according to Leviticus 18, God does not declare this wicked until the time of Moses.  As far as deformities are concerned, Adam and Eve were perfect beings made in the likeness of God and had no genetic deformities to pass down.  Only over several generations after the fall could genetic deformities have began to show. 
     Even without external sources, thoroughly analyzing the passages surrounding Cain and his wife helped me to understand what was going on.  As disgusting as it may seem to us today, he had to have married a sister, niece, or some other relative of his.  According to the story of creation in Genesis it would have been necessary to populate the earth, besides, what other choice would he have had?