Medical advances in Trek are so far ahead of us, as well as social progress, that I could easily imagine Trans people being so well integrated, and their transition so complete, that there's no reason it would ever come up. A main character could've transitioned as a teen and we'd never know and it wouldn't be an issue anyone would be bothered by.
The novels are quite a bit better with LGBT diversity. There are many prominent gay/bi characters as well as non cishet aliens, such as the Andorians, and Burgoyne in New Frontier who is dual-gendered and whose sexuality includes men and women. I don't recall any actual Trans people in the books. You could even argue that the trans gene is identified in fetuses and they get pre natal gender reassignment. If McCoy can give someone a pill to grow a new organ in minutes, why not? I am disapointed the new movies haven't had even a token inclusion.
That's the problem though. It's one thing to be talking about alien sexuality that people aren't really concerned about take The Cogenitor (ENT) with a society otherwise highly evolved strangely oppressing one of its three genders. When people watch this episode I am sure they are not at all concerned by it beyond the fictional drama because it's hard to relate to that sort of thing. But do they ever mention in passing that one of the members of the crew is gay or bi? A big fat NO on that one and yet it wouldn't be hard to do. Just a casual remark like phlox does now and then. When he asks trip if two straight crewmen that were about to have sex would agree to let him watch. I mean, they have no problem saying this CRAP and yet a single remark about two gay people is beyond their possibilities. Oh come on!!!
yes, the fact that it would be so easy to do and yet they haven't is an indication that such an omission is probably deliberate. Of course they can include lesbians in the mirror-verse for titillation purposes.![]()
Yes I believe the omission is deliberate. Gays in Trek was an issue regularly discussed, by the time we got to ENT discussed ad nauseum.