An easy example is insisting that trans people be allowed to play in sports based only on their identified gender. We have separation between mens' and women's teams because in many cases men have a biological advantage that would mean women did not ever win.

Insisting that an MTF woman be permitted to compete on equal footing somehow feels like offering equality to the trans person, but it does so at the expense of everyone else she competes against.

A perfect example is the MMF fighter who as a man was decidedly mediocre, but absolutely dominated when fighting as a woman. That is grossly unfair to the women she was competing against.

Trans supporters are fond of loudly, but falsely proclaiming anybody that holds this position is some kind of bigot. Except that biology is real and it is distinct from gender identification. Conflating them isn't helpful.