To be a prisoner means to be defined as a member of a group for whom the rules of what can be done to you, of what is seen as abuse of you, are reduced as part of the definition of your status.
So the idea that there is nothing essential, in the sense that there are no human universals, is dogma. Ask most anyone who is going to be shot at dawn.
Postmodernism is an academic theory, originating in academia with an academic elite, not in the world of women and men, where feminist theory is rooted.
Men who are in prison for rape think it's the dumbest thing that ever happened... they were put in jail for something very little different from what most men do most of the time and call it sex. The only difference is they got caught. It may also be true.
If you want to know who is being hurt in this society, go see what is being done and to whom in pornography and then go look for them other places in the world.
What postmodernism gives us instead is a multicultural defense for male violence - a defense for it wherever it is, which in effect is a pretty universal defense.