Setting aside that I personally feel “just because” is reason enough to include a trans character, isn’t the way you see this a problem for all kinds of characters aspects? I mean, why ever include a pregnant character, when not all audience members will want to be parents? Why writing about a married couple, when not everyone in the audience will want to get married? Why write a character that’s racist, when only a small part of any audience will be racists themselves? Why write characters as being from different parts of the world or from different religions, when the vast majority of your audience is not from that place or of that religion?
I don't think any of those things should be there unless there is a reason for it.
It's not about "if only some people want this", it's about being relevant to the story. No, I don't think there should be a pregnant character, unless it is relevant to the story. There shouldn't be married characters unless being married is relevant to the story. There shouldn't be racist characters unless their racism is relevant to the story. There shouldn't be characters from different parts of the world or different religions unless those traits are relevant to the story.
Or, at the very least, it might be more accurate to say there shouldn't be specific call outs to those things if they are not relevant to the story. I can include a black character, who exists and people know that he's black because he's black... I would probably NOT have a story arc focused around him telling everyone how black he is... it's why something like trans people are more difficult because it's more difficult to tell on the surface level. I know when a black character is black. I'll only know a trans character is trans if you tell me. And if you tell me something, there needs to be a story relevant reason for doing so.
I'm a huge proponent of "show, don't tell" in stories... and there's not really great appropriate ways to show that. It's easy to show a gay character is gay. Trans will almost certainly require dialogue, which should then be story-relevant.
I don’t see why “being trans” should somehow be a character trait that allegedly only there to “ticking off a diversity box”, when all kinds of character traits can and will only be part of some minority among the audience.
That's absolutely true. A trans character existing does not mean that character is there just to tick off a diversity box, but ALSO they could be, and maybe a bit too often today, are.
So I think it’s not unreasonable to expect that our stories also tell about these kind of experiences, whether it’s in a story set nowadays or set in the 24th century.
In most any other context, i'd say... yes.
In the context of Star Trek, the utopian vision of the future... no. We should beyond that. A trans character shouldn't even bat an eye in Star Trek. There's literally no reason for it ever even be brought up, nobody cares in the same vein of Uhura being called a racial slur by Space Lincoln, getting apologized to, and her reaction is basically "You said a word. Why would I care?"
Offshoot onto that, it was a weird issue in DS9, in "Badda Bing, Badda Boom", Sisko's reaction the Vegas program is odd for a 24th century person... from a person who definitely has not experienced any sort of racial discrimination in the slightest degree, being super against going to a recreation of 60's Vegas because the 60's was kind of racist? My dude, you're like 300 years removed from literally any sort of racism.