I’ve read about this before, actually… I don’t remember the exact wording, but it was something like that the patriarchy requires everything that a man does to hold authority over a woman, so penetration is perceived and understood as a form of domination over the one who is penetrated. The woman — read the one who is penetrated, as in patriarchy a man cannot be penetrated — has no agency in sex, so if you’re being penetrated you are not “having sex,” you are “being fucked” by someone else.