The belief that "Sons of God" (either fallen angels or created beings from other worlds) had sex with human women who bore offspring, and were then shut up in "the pit", comes mainly from The First Book of Enoch, an Apocryphal book. A very small portion of 1Enoch was quoted by Jude, and Peter alluded to it (or both were using an as yet unnamed source).
I very recently read through that book. I prayed earnestly first. Interesting read, to be sure.
The Nephillim were 300 cubits tall (450 feet). Really! How could they possibly have sex with women? In First Enoch, (and excuse me for being gross here) The Watchers = fallen Angels = Stars = Sons of God (used interchangeably) , "had penises like horses". (Wonder how large horses were back then.) How could human women possibly give birth to such large babies.
The Nephilim would live 500 years? Guess this was their lifespan - or the lifespan of their flesh. Then their spirits leave their bodies, and are confined in the place of conscious torment, until the judgment.
The "stars" which disobeyed the Lord are bound in the place "under the earth" "until ten thousand years, the time of their crimes, have come to pass". If they "lived" only 500 years, how did they commit crimes for 10K years? Doesn't make sense.
Another place says the spirits of wicked HUMANS go to a place of torment "whose souls are punished and bound there forever. Thus has it been from the beginning of the world."
Does not agree with my understanding that the dead are unconscious. No wonder the Catholics teach unending conscious torment in HELL. They include the Apocryphal books in their Bible.
Angels are presenting the prayers of the saints before God. I thought we have "one Mediator".
Writing with pen and ink was introduced by a fallen watcher named Penemue. This caused many to go astray. Really? Writing with pen and ink was evil? Personally, I believe that God introduced writing to preserve a record of history, of His law, and of His dealings with men.
In 60:1, Enoch is 500 years, 7 months, and 14 days old when he has a vision about judgment and sea monsters, etc. etc.
Genesis says Enoch was 300 when the LORD "took him" away, and he was never found afterwards. Does Enoch receive this vision after he is in the heavenly realm?
Enoch says the year is 364 days. Wrong. It's 365.25 days. If Enoch is passing on what he was "told" by the "angel" named Uriel, who "guides all the seasons and the heavenly luminaries of the sky" then Uriel was wrong. More likely, Enoch was actually written some time after 800 BC. That is the time when the extra days were incorporated into calendars around the world. Most probably a near earth fly by that pulled the earth into an elongated orbit. I believe another one will pull the earth back into a 360day orbit, so the end time prophecies can be fulfilled.
If the Book of Enoch was actually written after 800BC, then it was NOT the vision of the original Enoch.
12 "portals of heaven" - the sun is directed through a new one of those 12 "portals" each month. Doesn't fit with science.
81:1 "Enoch, look at the heavenly tablets" This one sounds like Joseph Smith. Sorry.
83:1-11 is Enoch's first vision. He writes about "the abyss". Seems he had heard of it. Wonder where? From demons.
86:1-6, 87:1-4, 88:1-3 The first fallen angel was named Azazel. Raphael cast him into the abyss. Really? I thought Satan was the first fallen angel, and Christ will cast him into the Abyss.
There are four archangels. There names are Michael, Raphael, Gabriel, Phanuel. In my Bible there is ONE Archangel (chief or head of angels). His name is Michael - the Son of God.
I will stop here. So many things seemed inconsistent with the rest of Scripture, or at least with my understanding of the Scriptures.