Nephilim influenced men to create the hybrid race (Satan's plan). Angels can not pro-create. They don't have those body parts. They were meant to be servants. The nephilim were chained (Jude 6 & 2 Peter 2)

God obviously knew that satan was trying to manipulate the bloodlines. In Gen 3:15, God tells the serpent that woman's seed (Jesus) would destroy him. So satan goes to try & manipulate.

There were millions of people on the earth before the flood. Only 8 made it. However, Ham (Noah's son) had a wife who was a Hittite. She came from Cain's bloodline. The race of hybrids survived through his wife, producing "giants."