So, I disagree with @r_speir, but his crazy idea is not so left field as you might imagine.
The position he is expounding is not invented by him. It is the story in the Book of the Watchers, which is part of the Books of Enoch, and are an ancient midrash on Genesis 6 from about 200 BC. This is actually one view of Genesis held by enough people. He is not inventing this. That is why you also echoes of this in Ken Hams imaginative diorama.
Before you knock it, it is worth understanding.
Now, I do not think this is what happened. I do not endorse this view (lest there be any doubt), but it does point to something about Genesis that we often miss. In my view, Genesis is full of ellipses, it's enduring appeal is in that it invites to wonder.