Let's assume it's accurate to classify being trans as a mental illness, just for the sake of argument. What if I told you we've tried pure psychological treatment and other medical interventions for trans people before - similar, interestingly enough, to L, G, B, and Q people* - and it is simply ineffective? What if I told you, after many years and unnecessarily dead humans, affirmation of identified gender, hormone treatment and surgical intervention were found to be the most effective means available for resolving the suffering inherent in the condition?

Let's also assume that despite this illness, a trans person is otherwise deemed fit and capable for rational decision making. You'd need evaluation by a psychiatrist to ensure this is true (which generally happens), but if so they have a fundamental right to bodily autonomy. Who are you or I to tell them what they can or cannot do to their bodies? Hell, if you right now wanted to surgically alter your dick to be comically large, if you're otherwise capable of decision-making (and a third party can attest to this), would you accept someone saying you can't, assuming you've accepted the possible risks?

End of the day, trans people are normal people who happen to have a disconnect between self-image and physical presentation based on hormone makeup and bodily form, and suffer quite a bit due to it. Transitioning is a game changer for most, and allows them to participate more effectively in the wider world without that baggage.

All you need to do is use a preferred pronoun, as community acceptance is a big, if low effort, part of the 'cure'. If you get it wrong, 99% of the time you'll be politely corrected. No one is going to crucify you for this.

Now, you can actively choose not to for whatever reason - but all that really does is make you an asshole in the eyes of many. Why? Because you're reinforcing something that causes another human pain, and what for? What does this accomplish for yourself, the trans person you're talking to, and your wider community, exactly?

I'm personally shocked this is such a hot button issue. Let other humans be whatever the hell they want to be, if they are competent to make decisions for themselves and 'changing' reduces their suffering. The world's burning, we have more pressing concerns than what our fellow humans have or don't have in their pants.

*A omitted simply because I don't know if asexual people were subject to anything like treatment for gay, lesbian or queer people at the hands of psychiatry and medicine more broadly historically. I imagine not, but I could be 100% wrong, and am without question uninformed.