Genesis 6 sons of God are human children of God, a term used several times in the bible referring to those chosen particularly by God, daughters of men signify otherwise. That verse is not about angels "spirit beings" mating with human women.
Genesis 6, the Nephlim were not men from outer space nor were they "giants" as in Jack and the Beanstalk. Nephlim means tyrant, brutes, in other words a bad old larger than life dictator. You want giants you have to go to the word Raphaim
I fully understand that is what has been taught in most Churches for a very long time. It's just wrong, for a lot of reasons.
People have in their minds what Angels are, and it is very difficult to get them to see the truth.
That fallen angels DID mate with human women.
The idea that the sons of Seth mating with the supposedly hotter daughters of Cain, (somehow the daughters of Seth were ugly) creating an unnatural hybrid race is patently absurd. "Spirit beings" don't eat. The men of Sodom wanted to have sex witha "Spirit being? Upon His resurrection was Jesus a Spirit being, or did He eat, and tell Thomas to feel His wounds?
The exact term used in Genesis 6 bene elohim, is used only three other times. All are in Job, All are clearly Angels.
Job 1:6
6 Now there was a day when the sons of God came to present themselves before the Lord, and
†Satan came also
†among them.
Job 2:1
2 Again there was a day when the sons of God came to present themselves before the Lord, and Satan came also among them to present himself before the Lord.
Job 38:7
7 When the morning stars sang together,
And all the sons of God shouted for joy?
Jude
6 And the
angels who did not keep their proper domain, but left their own abode, He has reserved in everlasting chains under darkness for the judgment of the great day; 7 as Sodom and Gomorrah, and the cities around them in a similar manner to these, having given themselves over to sexual immorality and gone after strange flesh, are set forth as an example, suffering the [vengeance of eternal fire.
Here Jude tells us that these angels that sinned are currently being held in chains, until final judgement, AND he tells WHAT the sin was, as he relates it to the Sin of Sodom and Gomorrah in going after strange flesh.
The Rephaim are descendants of the Nephelim. The Nephelim being distinguished as 1st generation angel/human offspring.
The Bible IS replete with giants. I can show you at least half a dozen places where they are encountered in Scripture if you like.