Cain and Abel? Just Cain and Abel? How was that supposed to work?

Okay, The book of Genesis says Adam and Eve had two sons, Cain and Abel. They grew to manhood before Cain Killed Abel. Okay, but say the murder took place. Cain and Abel lived to a ripe old age. How would they have populated the Earth. TWO GUYS? Even if Eve had daughters later on, that still wouldn't be right. Not only would that make the bible seem to encourage , but also pedophilia, as the girls would have been 18, 20 years younger than both Cain and Abel. Or at the earliest 16 years younger. I don't even want to think about anything younger than that!! The Muslims have that covered with Mohammed's 9 year old wife!!!