Yes, there is a reason. woman -> femina, related to women -> femininus man -> vir, related to men -> mas As you can see it seems even the Romans did not like

Yes, there is a reason. woman -> femina, related to women -> femininus man -> vir, related to men -> mas As you can see it seems even the Romans did not like to make derivations from the old, old word "vir" (also see etymology for "world"). Feminine is from 'femininus', not 'femina'. Masculine is from 'mas' -> 'masculus' (diminutive of 'mas') -> 'masculinus', not 'vir'. 'Mas' was never used to mean "man", only "male". If instead of "feminism", the movement were called "femininism", then the opposite would indeed be "masculinism" as you suggest. But the opposite of "feminism" must be "virilism".