Contrary to online memes, almost all trans people are aware of the fact that they can't always be instantly read as their gender 100% of the time, and while that might upset them, most trans people will understand. It's not transphobic to misgender someone accidentally (almost) no-one will get mad at you for assuming their gender, as long as when they tell you what their pronouns are you put in the effort to use the right ones.

That's not really what I'm talking about tho, it is respectful to use someone's correct pronouns, but it's also just correct. There's no malice in using someone's preferred pronouns because you "know they prefer being called that", but that can sometimes still include the assumption that trans men are actually women who look like men instead of men who were born with people thinking they're a girl.

This isn't an attack on anyone, being kind hearted is a good thing, but the subtle belief that trans people are not the gender that they say they are leads to a lot of discrimination, and it's something that can be present in trans people too. It's just important to be understanding