The proof is that men aren't described as gatekeepers, even though just like women, they choose to have sex with certain people, and not with others.
The "gate" you're talking about uncontroversially belongs to each individual in the form of their bodily autonomy. Yet women are uniquely cast as "keeping out" certain people. That's not a matter of dimorphism: that's a matter of mens' bodily autonomy being assumed, and womens' being up for debate.
In other words: for the sake of argument, sure, there's a gate. But only women are viewed as gatekeepers, as though keeping others out undermines equality, rather than being part-and-parcel of having the equal autonomy to choose one's partners.
The "gate" you're talking about uncontroversially belongs to each individual in the form of their bodily autonomy. Yet women are uniquely cast as "keeping out" certain people. That's not a matter of dimorphism: that's a matter of mens' bodily autonomy being assumed, and womens' being up for debate.
In other words: for the sake of argument, sure, there's a gate. But only women are viewed as gatekeepers, as though keeping others out undermines equality, rather than being part-and-parcel of having the equal autonomy to choose one's partners.