Indeed, it states the “sons of God” knew the “daughters of men”.
I have no problem with that at all. it is the leap to assume that the sons of god in this text actually means outcast fallen angels/demons who were created, not begotten and therefore not sons at all.
Things were very different before the flood. In the New Testament, Jesus is referred to as His “only begotten son”. That might be construed as He considered His other created beings sons, of a sort. There was a different covenant in force then. I am far from an expert, and not adequately equipped for a deep discussion on this, but I do enjoy the discussion, just the same.