From Bart Ehrman’s post here.
One of the most mysterious and even bizarre stories in Genesis happens right at the beginning of the flood narrative, where we are told that the “sons of God” looked down among the human “daughters,” saw that they were beautiful, and came down and had sex with them leading, to the Nephilim. The word Nephilim means “fallen ones.” According to Numbers 13:33, the Nephilim were giants. So what is going on here in Genesis? Apparently there were angelic beings (the “sons of God”) who lusted after human women, cohabited with them, and their offspring were giants. It is at that point that God decides to destroy the world. The situation was too weird even for him.
[Gen 6:1-4 ASV] 1 And it came to pass, when men began to multiply on the face of the ground, 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 that they chose. 3 And Jehovah said, My Spirit shall not strive with man for ever, for that he also is flesh: yet shall his days be a hundred and twenty years. 4 The Nephilim were in the earth in those days, and also after that, when the sons of God came in unto the daughters of men, and they bare children to them: the same were the mighty men that were of old, the men of renown.
