1 And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,
2 That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they chose.
What if Neanderthals were the superior human? They had a slightly bigger braincase, more muscular, better able to survive in the cold climates. But what if they found homo sapiens women more attractive than Neanderthal? They would preferentially take homo-sapiens women as wives, who would not be able to produce children for them unless the women mated on the side with homo-sapiens men