Lighter and Theland, from my understanding you say you take the story of the creation literally. In that case, two different types of man were created. That's what the bible says.
However, if we go along with your assertion that Adam was the first and only man created in the beginning, and from there on take what's written as the literal truth, as you say you do, there were only three people on earth when Cain went to the land of Nod, so he couldn't have taken his wife with him. So, where did Cain's wife come from? She couldn't have been a relation because Adam and Eve's other children arrived much later in the story - long after Cain's wife had given birth to Enoch - and the chronology can't be at fault because, according to you, the bible is the literal truth. And just to add to the problem we're told that after Enoch was born, Cain built a city. A city? A city needs a population.
If you believe the bible to be the literal truth, your assertion that 'Cain inevitably married a sister (or maybe niece)', cannot be so, and therefore, you are in reality making the story up to suit yourself.
Frankly, I think I take the bible more literally than you because I read it as an historical document and my vision is not blinkered by religious dogma. I believe there are clues to our past in the bible, and in other ancient texts, but clearly you see what you want to see, twist it to suit yourselves, and ignore what's really there - and the bits you don't like.
Incidentally, Cain didn't exile himself. He was 'driven out'. (Gen 4:14).