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Man would have done that with or without the angels. Notice no mention of the angels, only man.
You don’t find it kind of coincidental that out of the two times nephilim are mentioned in the entire Bible it’s both in a negative context? Out of every chapter in Genesis God picks chapter 6 that deals exclusively with the wickedness on the earth to bring this topic of human angel relationship up? Isn’t it more likely it was included in chapter 6 because of it being included in that wickedness?