That’s one of the most confusing few verses in all of scripture. I think the flood had to happen to preserve the line Jesus was to be born into. If the nephilim were not destroyed in the flood, it appears from scripture, they were corrupting man and its lineage to the point it was going to completely self destruct. I have a hard time comprehending the possibility of human/fallen angel sexual relations, if it’s even possible at all. Apparently angels look like men, and whether they function like men I obviously do not know. It’s worth thinking about, but this side of eternity I don’t think we’ll know.