No one is suggesting that opposite sex negative things (cliques, shaming girls based on clothes, whatever other things they do) is acceptable.
But I've yet to hear anyone complaining about boys being intimidated and not participating in a class because it was full of girls being bitchy. And to the extend that it happens (someone mocking an ugly kid in class), I'd assume it'd get slapped down in the same way.
The "masculinity-affirming posturing and violence" that the parent refers to is pretty much entirely negative. There's no benefit to praise, accept, or encourage that kind of behaviour.
