According to Scripture, Adam and Eve had three children, Cain, Able and Seth, who married and had children. Whom did they marry?
Adam and Eve had both sons and daughters (Gen. 5:4). Because there were no human beings except those born of Adam and Eve, sibling marriages were a necessity. St. Augustine says,
As, therefore, the human race, subsequently to the first marriage of the man who was made of dust, and his wife who was made out of his side, required the union of males and females in order that it might multiply, and as there were no human beings except those who had been born of these two, men took their sisters for wives,—an act which was as certainly dictated by necessity in these ancient days as afterwards it was condemned by the prohibitions of religion . . . and though it was quite allowable in the earliest ages of the human race to marry one’s sister, it is now abhorred as a thing which no circumstances could justify. (The City of God XV.16)