Player doesn't respect NPC pronouns

This isn't really a horror story, and not even really that big of an issue. It's mostly just a vent/rant about a mild inconvenience at best, that I honestly, shouldn't be bothered by.

As a DM, I like to take pride in the world I've created and the characters that fill it. And I wanna be clear--the player, and whole situation isn't a big deal, it's just mildly frustrating to me.

Anyway, when I'm designing significant NPCs, I like to give them personality, motivations, backstories--and reasons to keep going. I very much get attached to the characters I make with the goal of my players getting invested in them too, from quirky shopkeepers, tragic heroes, to complex individuals stuck in a morally ambiguous world. That way--when something happens they genuinely feel for them.

While I'm not exactly one to force LGBT agendas or diversity quotas down my player's throats--I do like to have some kind of representation of different personalities or individuals from time to time to flesh the world out a bit more, and make certain characters a bit more unique.

One such character is an NPC I created called "Henry," but, they prefer to go by "Taylor." Taylor identifies as Non-binary and prefers to go by They/Them, but doesn't mind going by He/they. And for simplicity, Taylor's twin sister, Tally, still calls them her brother. Taylor's entire schtick is they want to be independent of their twin--but because of their complex childhood trauma they suffer from crippling separation anxiety when away from their sister, Tally.

When I'm describing characters, I don't say "This is Taylor, they are non-binary, and go by They/them" or "this is Susan, she goes by She/Her." I try to imply identity through my prose.

I know this is incredibly inconsequential and these characters are not real, and just imaginary, but--I--prolly like other DMs or writers want the characters to be taken with a semblance of seriousness. When I present a character, I expect the party to give them the same respect as I do their characters. And the player in question isn't blatantly misgendering the character considering they don't mind being referred to as a guy or other masculine terms, or nor does the player have any malicious intent with their remarks.

But in all of my prose, I've referred to This NPC as They/Them, and have only described Taylor as a "he" for narrative consistency to differentiate between characters (also part of the reason why I kept that as part of their personality). The party also has copies of Taylor's medical records (From a quest they recently completed) that explicitly state their gender identity is Non-binary. Everyone else in the party when they interact with Taylor refers to them by their preferred pronouns. But the player in question doesn't. Not for a lack of trying, mind you, and more of they just have a hard time wrapping their head around the idea.

Again, this really isn't an issue. It's not even worth talking to the player over, because its such an inconsequential detail, this is just me bitching into the Ether just to hear my own voice at how stupid I sound. I know I'm prolly just overreacting and absurdly flustered, but IDK.

That's it.

TL;DR: I made a Non-binary NPC, Player unintentionally doesn't respect their pronouns, I'm overreacting over such a tiny detail. Boo-hoo woes-is-me-I'm-a-professional-DM-and-your-ruining-what-I-created.