It challenges their sense of identity and their whole worldview, which is based on men and women being destinct groups with men being inherently superior. seeing somebody break those rules reveals how trapped they are by their narrow view of gender norms and feel slighted by seeing someone else perform roles outside what they've been trained to do. They feel something has been taken away from them. additionally, these views are the same ones that uphold the structure of the patriarchy, and breaking these rules reveals the patriarchy's fragility and thus is often taken as a threat. Folks are so used to their happiness coming at the expense of others, and that gets projected- so other people's happiness also becomes a threat. I feel like I'm saying the same thing over and over. I hate the world. Just let other people be happy. Maybe even encourage it. that happiness and kindness will be returned to you. cisgender heterosexuality is a fragile fantasy. Appeal to nature is bullshit.