Imagine I read your comment and didn't think of a trans person, but a Ms. Jones who had married a Mr. Smith and was now Mrs. Smith.

Is your opinion the same?

Yes.

Marriage is a union where two people make a lifelong commitment to each other: their identity is now united. This is a normal and good thing.

While it's hard to shift, as long as you're making an effort it's fine. If you slip up from time to time, especially early on, that's understandable. But eventually you should learn it. You were willing to call them by what they asked to be called originally, rather than just calling them "big guy" or whatever, so I don't think it's unreasonable to respect their wishes on what they wish to be called in the future too.

There is a big difference giving someone a nickname vs. assenting to a reorientation of outward identity.

Someone wants a nick-name? Sure. No problem.

Someone who was a guy/gal and now claims to be something else? Especially if he/she is actively known in a space as something before and _then_ changes? That's a HUGE demand that honestly shouldn't be something anyone is comfortable asking others to do.

I do this too, but with people who change their phone number. No, that number still exists, and I associate it with them. They choose to change it? That's their problem.

Edit: Guess I should put a /s. Is there a tag for highly passive aggresive /s?

Perhaps it should be /sa should be for /[s]trawman [a]rgument.