I accept Gen 6 as it is, but many theologians don’t (for reasons that seem odd).
I think it has to do with sex. Most everyone I have ever heard are traumatized by the idea of angels and sex. They are supposed to be sexless. Jesus did say they did not marry. However, how can a sexless angel view a beautiful woman and be sexually tempted if he is sexless? Or does not have the capability?
There is a mention of two female angels in the Old Testament, but the commentators rush to inform that it is purely symbolism, etc., and cannot possibly be literal.