What is it called when you're against transgender people?

if you are against trans people, then that usually means you are agaisnt the lgbtq community, which they are a part of.

When you are against transgender people it is called transphobia - not because you are scared of trans people necessarily, but as you're so deeply invested in your ignorant ideas about sex and gender you're scared of the fact transgender people represent a challenge to those ideas. Otherwise we would call those people bigots, prejudice, ignorant, or straight-up assholes.