I've seen this asked before, and I think for a lot of people for whom transgenderism was a bit of a culture shock feel, they feel like one day we called them "trannies" and it was fine to make fun of them in shows like Night Court or whatever, and then boom!- they're a protected class and it's a bit of a mental hurdle people need to figure out. Asking questions when you're not sure is A POSITIVE STEP.

I AM NOT A PSYCHOLOGIST, but from what I've read we typically only assign the term "mental disorder" for things that negatively impact people's live. The negative impact for trans people comes from others, from a lack of understanding or care, so that makes what they endure a social condition not a mental one.

The World Health Organization changed their position on being transgender this year, and now consider it a "sexual health condition" to acknowledge the need to access necessary health interventions but reduce the stigma. This is not too disimilar to the journey being gay went on in the mental health community: moving from a symptom of mental illness ("this MAN is sexually attracted to men and not women, there must be something wrong with him") to a sexual condition ("he happens to be gay").

And although it's a really sloppy way to do it, and I just mean this for the sake of the metaphor, maybe thinking about it the way you think about homosexuality will help you understand better. Most people acknowledge that people just born gay, it's who they are, they didn't choose it. Similarly with trans people this is not a choice they're making but something that is inherently true TO THEMSELVES, about THEMSELVES that they're allowed to make manifest in the world. That make sense?