Thoughts on origin Nephelim?

I've only heard of two theories on the Nephelim that I think are plausible and not alien-cope.

The Sethite theory, which posits that the "sons of God" were the sons of Seth while the "daughters of man" were of Cain.

The other theory (I forgot the name) that the "sons of God" were fallen angels while the "daughters of man" were humans. One thing that I heard from a priest once that there was a ritual for this where a king would be inhabited by a demon while mating with a ritual prostitute which explains how a demon, which has no material body, could have offspring.

Any other theories I am missing?