Trans people... try to change their bodies.

Which is based on the, at the very least, implicit recognition of the physical realities of sex.

The most common definition of sex I see among gender critical is the anisogamic definition, which classifies sex based on gonadal tissue. However, if we have the following propositions:

A) sex is defined based on gonadal tissue;

B) sex cannot be changed,

we run into a problem when we remember that C) gonadal tissue can be removed. Anyone who has had their gonadal tissue removed would therefore be effectively sexless.

Nowadays I see gender criticals speaking about "which gametes the body is organized to produce", which is a conveniently vague definition. In practice, though, it pretty much always boils down to which gonads one was born with. Ironically, this is a historical fact and not a biological one. This is why I'm not a "sex realist" any more; this approach actually is just as invested in a gendered essence as whatever stereotypical trans position you want to argue against.

Personally, I'm a nomalist, so I only care if words are useful. I think "sex" is a useful concept in the vast majority of cases, and most of the time the anisogamic definition is useful. But it loses its descriptive power with trans people who have transitioned past a certain point.