The term “boy’s club” here is really not generalizing “men” or “boys” as a whole, but rather it’s by its usage criticizing the specific group mentality it describes, that of a group of “boys” who treat women with less respect than each other, or otherwise exclude said women, as in at least some cultures is common from some generally younger “boys” who haven’t really matured past a mentality usually developed from a young age, because they lack the experience to know it’s wrong.