Even in the first one, ‘boys will be boys’ is often used to explain why you react differently to the same scenario depending on whether it’s a boy or girl. I remember being scolded for this stuff as an AFAB, while boys got away with it. Same story with getting into little physical fights, being rowdy, aggressive, destroying stuff. I guess that can be a slippery slope into the latter one, if you keep it up long enough. Just my attempt at an explanation.