I personally believe that cis people have a baked in concept of gender fluidity that they associate with gender trauma. Being socialized into patriarchal gender involves a lot of being told that you are a deviation from the platonic ideal of your assigned gender. So you will be punished with androgyny or have your Proper Gender status revoked from you if you don’t meet certain expectations. People understand, even if they don’t agree, that there is an ideal man and an ideal woman, and that when people point out the ways that you deviate from those ideals they are being mean.

Trans people threaten the idea that gender can be a universally agreed upon ideal which is determined immutably at birth and is easily signaled to others for the sake of signaling conformity. And a lot of cis people haven’t attempted gender nonconformity since before they could write a full sentence, so they have processed none of that patriarchal conditioning in decades.

I don’t wanna write off all transphobia as a trauma response because that’s a cop out on many levels as well as inaccurate for many people. My point is that cis people understand gender deviation. They just understand to be bad and couldn’t articulate a historical reason for all the pain that taught them that