Just to throw up some bio to give my remarks context: I’m 54, straight, cis-male. The first novel I really remember reading was Harry Harrison’s Spaceship Medic when I was around nine-ish, I think? Though I’d been a keen fan of sci-fi comics before then (TV-21 is the only title I recall from that time).
I’ve got a feeling that the first transgender (human) character I read about was probably the protagonist of Heinlein’s I Will Fear No Evil. I don’t think it’s a spoiler to reveal that he gets a whole body transplant early on, transferring his brain from his old man’s body to that of his pretty young secretary’s (who coincidentally has a fatal-to-the-brain accident around the same time as he has a heart attack).
Now, for someone whose undead corpse gets held up as a totem for conservatives all the time, Heinlein had some pretty liberal ideas, not just about sex, but about gender, too. By our standards he may have failed on the follow-through in real life, and even arguably handled them clumsily in his fiction, but he had no difficulty in contemplating what was originally a man’s brain in a woman’s body and accepting that she was now a woman by any useful definition.
I think our present-day conservatives would make a great deal of that, probably falling on the side of insisting that our protag is still male and getting the screaming habdabs at contemplating “him” having sex with a man. (Which she does, rather a lot, as I recall. (But discreetly written. (Still, it was pretty steamy for my pre-internet 14yo brain.))
I think that, though I’m far from perfect in this regard, to the extent that I’m accepting of trans* issues today, it’s mostly down to reading such out-there stuff early on. For a long time I thought what the general media wanted me to think about it all, but eventually (fairly late in the day, really), rejecting the general narrative wasn’t such a big step because of books like this.
Saying that, IWFNE does rather weigh the considerations heavily towards the sexual end of gender issues, I think, which is one of the clumsinesses of Heinlein’s text. It could definitively been better in this regard. But from my young cis-gendered perspective, I’ll give it a C+.
(Is this the kind of thing you were wanting, @jerwin ? )