The angel theory doesn't make sense to me, as angels evidently don't 'breed'. The idea that the daughters of men were the descendents of Cain is possible. I've sometimes wondered if maybe Daughters of men = Neanderthals.
The idea that these were angels can be found in the Book of Enoch, which although isn't canonical is referenced to in the New Testament. I'm not sure how much of the book can be trusted, what little I've read of it is pretty wild.
Possible...all possible. But it just doesn't add up.
If the daughters of men were the descendents of Cain then how does that make everyone else the sons of God?
Man was created from the elements of the Earth...and woman from man.
This doesn't seem a likely creature to be called the 'sons of God'.
Men were men and they had daughters. The passages in Genesis speak of giants who were called the sons of God...the Nephilim. To me, this speaks directly of a different species...and it creeps me out when I think on it too long.
It's almost a little too Erich von Danikenish.