ASV notes on Genesis 6:1-4
This passage in the Bible has been frequently omitted by scholars, here is my opinion. Who were the “sons of God”. These men were run by their flesh, that is by their 5 senses and their feelings, they looked at women with desire and lustful imagination. They were not governed by the Spirit of God. The Spirit of God is always against the flesh.
So these men were not of God spiritually, but could have been demon possessed men because fallen angels cannot replicate by having children as far as I understand. It is said in 6:4 that the men also lived afterward; it could mean that they continued to live after the flood, not the men in flesh, but men being possessed by the same demons.
The men and women growing up with Noah lived a corrupt life given over to sensuality like the development we experience today; especially with all the nakedness and sexuality on TV pushing the borders of what is normal and legal.