In reply to those who were confused as to why I have a strong aversion to they/them pronouns - there are two answers. The primary driving reason is the emotional one, so I'll cover that first.
1. I don't particularly care about anyone's gender (unless they're a romantic or sexual partner, in which case it's relevant). I don't know how many genders there are, but it's at least three, and I've seen claims that they number into the dozens. I don't have the time or interest to learn everyone's gender. When I use he/him/she/her pronouns, I use them in their gender-neutral forms. My use of pronouns is simply a reflection of the perceived sexual characteristics of the person I'm referring to. NOT their gender.
I don't think I'm weird in this. This is the societal default. It's why tomboys retain the she/her pronouns, and fa'afafine retain the he/him pronouns.
Yes, it is dumb that our language has different pronouns for apparently-male-sex people and apparently-female-sex people. It's dumb that our brains have different specialized slots for apparently-male-sex people and apparently-female-sex people too, but there it is. When I was young and my brain was being molded, the language parts of my brain were hooked up to the sex-recognition parts of my brain via methods that have been refined through cultural evolution to hook those two parts together very strongly. And it took.
When one insists others use pronouns that contradict with the one's sexual presentation, I am required to overrule my own lying eyes and instead use arbitrary terms picked by that person. It feels like I am being told there are five lights every single time. Last time it was my church and parents who were telling me there were five lights. Now it's my friends. :( I am being forced to lie every time I speak of them, and I despise it.
This is bad enough on it's own! But in addition...
2. Misgendering suffers from Lie Inflation. Many trans people suffer from dysphoria, and successfully transitioning is an intensely laborious task that takes years of effort, and usually major biological intervention. And since perceived sex is socially mitigated, how people are treated can make a big difference to perception for those who are on the borders of passing. So intentional misgendering can be really harmful. "Misgendering" someone used to be the term for a malicious attempt to drag people backwards in their transition.
Of course, if you know your friend is trying to get better at something, the polite thing to do is to act like they're already good at it. This is why writers can never trust feedback from friends and family. It is polite and affirming to use the pronouns that go with the sex someone is hoping to be seen as. So naturally the term "misgendering" has in time been inflated to include people who are unwilling to deny that a dude with a beard has apparently-male sexual characteristics. As a result, if I don't constantly monitor myself I am in the same moral ballpark as the fundamentalist who is maliciously tearing apart the years of work of trans people.
And yes, my friends are kind and supportive, and they "forgive" me when I slip up, because they know this is a hard thing that takes a lot of effort. No one is about to disown me (I think), they just keep dropping polite reminders. But inside I am seething, because I don't need forgiveness for accidentally blurting out that There Are Four Lights. I'm jealous of the people in my social group who haven't yet been told that Person X is a Them now, because no one judges them poorly for using the obvious pronouns. I sure as hell won't ever tell them, because I don't want to the the jerk who has permanently imposed that cost on them. Honestly, if I would be better off not knowing someone's mystery gender, I wish they simply wouldn't tell me their gender.
3. This is where I came to see the parallels with my earlier life. I grew up with abusive relationships. As is typical, I recreated my past, so I was in several abusive relationships as an adult as well. A constant in nearly all abusive relationships (and certainly every one I've been in) is that the abused party is constantly monitoring their behavior and speech around the abuser so as not to set them off. The common phrase is "walking on eggshells." Mistakes are rarely punished, of course, but that randomness makes things worse, because you can never be sure you're safe.
The constant monitoring of my speech to not ever slip into using words that match the perceived sex in this one particular case invokes that exact same feeling. Never has anyone exploded on me for failing to use the neuter pronouns, but of course that just means it'll be even worse once it does happen, according to my brain. Perhaps I could use this as evidence to slowly move away from this fear, if it wasn't for the fact that some of the neuter-gender people I personally know have publicly announced "If you can't respect me enough to use the right pronouns, I don't want you in my life."
This wraps up the emotional reasons for hating they/them. The lesser reason is a practical one - there currently is no neuter-sex, and trying to create one in this manner does social harm that isn't worth the cost. But that'll be a post for later in the week.