Man cannot have sex with angels the two realms cannot interact in that way. So why do the commentators persist in attributing this to man's lifespan being reduced too 120 years? It is because the book of Jude alludes to this but what Jude is referring to is that women are fallen angels and in their physical state sleep with men, because they are still eternal beings. In Genesis 6:1-4 the years were cut down because mighty men were stealing other men's wives.