Preface: I only thought to come back here because the most recent Regened thread just got co-opted by someone unable to keep their FART (Fairly Aggressive Rowling Tirade) tendencies in check.
Before anyone explains how shit she is - please, you don't need to, because I've already heard the arguments. (Aside: if, in order to understand your point, I have to sit through 2.5 hours of Internet naval-gazing, then - well, you're asking too much. Start with a five-minute summary. Seriously.)
Here's a puzzler - someone I know is non-binary, but also really enjoys the Harry Potter books. I'm hoping they can continue being happy with both those aspects of their life, without some foam-mouthed Herbert insisting that the author of the thing they enjoy isn't worth spit. Because here's the thing: it's a bit more complicated than that.
All this business about people being on a spectrum is very good - I like that. Gender spectrum. Mental health spectrum. All good. For some reason, though - it seems like people are quite quick to judge (especially celebrity-status) folk they're having a disagreement with as either all bad or all good. What happened to the complexity of a spectrum all of a sudden?
TERF belongs in the same bin as woke. It's just an attack-term that devolves any discussion into two sides of a fight. That's fine if you just want to fight. It's not much good for trying to reason with one another.
The way I see it, adopting the form of address someone requests costs nothing and is nothing more than common courtesy. Doesn't really matter if it's gender, rank or qualification.
I'm not saying you're wrong. The thing is: remembering someone is Doctor McCoy, and not just Mr. McCoy - that's pretty easy, because it's not battling an existing, ingrained, verbal-reflex that's not even part of my conscious thought process.
Or, a guy called Kenny doesn't like to be called Ken. Easy. I might fuck up once or twice, but probably not. Kenny likes to be called Kenny. Doesn't care for Ken. No problem.
Someone who looks like a female, to all my senses, running on automatic (in the same way that I recognize a dog, or a tree, or a car, without even thinking about it) wants me to use different pronouns. Sure, I can make every effort to do as they ask, but it's battling an ingrained, innate sense that seems to a great extent beyond my conscious control. So, I find myself accidentally misgendering folk. I do all the right things after that - I correct myself, apologize if it seems appropriate given the social dynamic, and so on. I try to get it right. I really do.
But it's not as easy as Doctor McCoy and Kenny. My point is that it being "common courtesy" suggests that it's as easy as holding open a door for someone. It's not that easy. There's a battle of new language, new social expectations and new concepts - that are up against half a lifetime of social and linguistic pressure.
I tried to write a character once in some fiction, where I didn't want the reader to know their gender until a later reveal. So, I was avoiding he/she or his/her, and naturally thought to use they/them. But, sometimes that would bring up an obvious ambiguity with the idea of pluralism, so I'd have to think about restructuring the entire sentence. That's okay when you're writing a book - but it's a bit awkward if you're having a conversation.
Our language may evolve to cope with our new societal views on gender identity, but it's difficult to force language to do one's bidding.