Then you are perpetuating cissexism.

... how so?

there is no brain scan for trans

The fact that we cannot currently diagnose gender dysphoria [EDIT: in a living subject] with a brain scan does not change the fact that it is caused by a neurological disorder, and as such is biological, not a choice.

if you experience yourself as trans and the scan says "nope" it's the scan that's wrong. The individual is the sole authority and the diagnosis is by telling a shrink what you experience.

Are you saying that cisgendered people should be eligible for gender reassignment surgery and so on, or that any brain-scan based test will be imperfect?