In the first place Catholics do not believe in the existence of pre-Adamite people, but rather believe in the unity of the human race descended from Adam and Eve who were the first parents.

It was not the intention of the inspired writer to name all the children of Adam and Eve. As a matter of fact they did not. The Bible names three sons, Cain, Able and Seth, and then it goes on to say that Adam "begat sons and daughters." (Gen. 5:4).

Therefore, Cain married one of his sisters. This marriage of blood relatives was, of course, necessary in order that the human race might be begun.