They are, just not nearly as much as for women. When I was at school it was a thing for girls to call the "player" boys manwhores or sluts, because they thought it was unfair that only the girls had their sexualiry policed in such a way. It wasn't just a 'joke'.
Obvs not going to disagree that it's gendered, just that it can be used against men if you take the definition to be having loose sexual morals. Personally I don't think being 'promiscuous" means you are less moral than someone who isn't, but I think cheating does, and that's where I'd use "slut".