I tend to think the theory that Nephilim were a result of the fallen angels mating with human women makes sense. Yet, I am still confused by the fact that Jesus Himself said that angels do not marry, nor are they given in marriage in Matt 22:30 while Genesis 6:2 explicitly states that the “sons of God” married the daughters of women. And the fact that even if they did possess a human male for this process rather than just manifesting themselves as a human, how could the angelic part of the Union be passed on to offspring if life reproduces after its own kind according to Genesis and yet the Angels don’t reproduce?
The angels who fathered the neplilim disobeyed God. First by taking on human form when they were not authorized to do so, and second by fathering children with human woman, producing a hybrid race. In order to do this they couldn’t simply possess human men. They had to abandon their natural state and actually appear human-like. Somehow they were able to make their DNA close enough to human DNA to get the women pregnant, but were still able to pass along the supernatural traits for which the Nephilim were famous.
I also believe the phrase “they married any of them they chose” (Genesis 6:2) means they all took multiple women for the purpose of procreation. So I don’t believe it was a marriage in the way you and I think about it.
This is why they’ve been held in gloomy dungeons (2 Peter 2:4), bound with everlasting chains (Jude 6) awaiting their day of judgment.
When Jesus spoke of angels not marrying He was talking about those who did not commit these offenses but obey God’s rules.