Fighting sports rules
I'm sure everyone knows that in any fighting sport there's an important rule: hitting the balls is not allowed. Even in MMA where this guys get hit in their faces really hard all the time there's a specific prohibition of any groin attack. Imagine how embarrassing it would be watching this big muscled guy grabbing his nuts in pain after just one hit. Imagine how embarrassing it would be for all male gender if they change the rules allowing it and making it mixed, a big guy totally defeated after just one hit of a smaller women, and can't hit her back because she doesn't have this humiliating weakness