Cain's wife could of also been a human like creature he found after he left his home town as explained here:
Who was Cain's wife?
I know a lot of people say that Cain's wife was his sister, but that is impossible. Why? Because if you know anything about Anthropology, you will know that in all primitive societies it is a general rule that brothers do NOT marry their sisters. The strictest of taboos are applied to this particular form of incest. Secondly, if you read the story carefully, Seth (the 3rd son) was not born until after Cain was banished, and thus after he spoke of EVERYONE in Genesis 4:14.
After Seth was born, Adam lived another 800 years, producing more sons and daughters. This obviously shows that there was Cain, Abel, Seth, and THEN other daughters and sons. Thus the explanation that Cain's wife was his sister cannot be correct, because females did not enter the population until after Seth, and by that time, Cain was already banished.
So who did Cain take for his wife?
You need to analysize the story of Eden. According to Scripture, what did God do? He planted a Garden in Eden. Note: the entire earth was NOT Eden, no, only a very small portion of the earth was Eden. What does the Bible say was outside of Eden? Desert and wilderness. And WHO lived in this wilderness? It was Australopithecus, Homo-Erectus, Neanderthal man, and Cro-Magnon man, who roamed the wilderness outside of Eden.
These creatures were very tall, broad-faced, had retreating foreheads (indicating a small brain - unlike humans today) with massive brow ridges, and their bodies were covered in very course dense hair. Note also that Cain was fearful to leave the place of his birth, because he knew there were other people out there who would kill him. Know don't you find this very strange considering that God marked him as the "first murderer?" And how did he know they would kill him? Because this type of very violent behaviour was typical throughout the species of Australopithecus, Homo-Erectus, Neanderthal man, and Cro-Magnon man.
So this is where Cain got his wife from. She was one of these very primitive "creatures" roaming the wilderness. As a result of this successive hybrid inbreeding over many generations, we get what we call the "other races." ie. Negroes, Orientals, Red Skins, etc.