Reducing the Spectrum to a Binary
As promised, here is why I think they/them pronouns are more harmful than useful.
Up through the 2000s, we were making good progress on diversifying the sexes. Gender was coming to be understood as more of a spectrum. There were many ways to be a man. You could be a drag queen or a bro. You could be a stay-at-home-dad or a metrosexual. Being gay or straight didn't even matter anymore. Sure, there was still some toxic masculinity enforced in various hellholes, and lots more internalized toxic masculinity everyone was trying to get over. But it was accepted that there was no one script for "manliness" anymore.
Women, of course, have always had multiple options, and as men's options expanded, women's kept pace. Dozens of TV shows and movie roles explored the myriad ways one could be a woman, and there were role-models galore.
And somehow our progressive movement managed to take this spectrum and cut it down to just three options. Just last week I saw a friend bemoaning "a binary culture which only allows masculine males and feminine females." The new dogma is that there exists only this binary, that we've only ever had this binary, and that if you don't think of yourself as a He-Man Woman Hater or a Barbie Doll Girly Girl you are non-binary and should adopt a neuter-sex position.
This is stupid. It erases all the people who've come before who pushed the boundaries of what it meant to be male and what it meant to be female. The people who made it OK to be a guy and cry without crippling shame. The people who made it OK to be a woman and like casual sex, or heavy metal music.
It also tells everyone who doesn't identify as neuter-sex that they must adopt the traditional ultra-masculine or ultra-feminine roles or they aren't really part of that gender. This is almost exactly the same message that the assholes had been preaching before. This is a regression. When someone says "I'm not the kind of person who enjoys slamming back beers and hitting on random chicks all night" and someone else tells them "There's a word for that! It means you're non-binary!" I die a little inside. I guess that, since I was born with a penis and I don't ask people to deny that fact with awkward pronoun-usage, I'm just like all those chads. That's great.
Obviously there's no reason our language needs to have gendered pronouns. But inventing a neuter-sex and trying to shoehorn people who aren't inter-sex into it is the opposite of a good way to reform the system. That's adding complications rather than removing them. Since so few people are inter-sex, this neuter-gender can only be filled by creating a false gender-binary and offering the only alternative. This is not so different from creating a false "original sin" and then offering the only absolution. And since the invented neuter-sex doesn't carve reality at the joints, its use can only be enforced with shame and social ostracism... which will make these reforms deeply unpopular even among the sympathetic.
If one wants to make our language gender-neutral, one would be advised to stop using gendered language themselves, rather than trying to create a neuter-sex and require others to contort their thought-processes around it. At least as a first step.