The fact that no one ever debates what the definition of a “man” is says it all about the recent UK Supreme Court judgement.
Transphobia is deeply rooted in misogyny.
It’s the idea that women should look a certain way, based on sexist ideas of female sexuality through the male gaze, and should act a certain way, based on a patriarchal belief system.
It’s the idea women are only good for procreation and satisfying men sexually.
That’s why we never hear about trans men.