When people try to give me crap when I say I use they pronouns I always respond:
“I’m a linguist. I’ve studied how language works thoroughly in Spanish and Latin as well as English and I assure you, there is nothing grammatically incorrect about shifting the use of the singular they (something that has been in use since before shakespeare!!) from the indirect to the personal – language is not bound by rules but a fluid thing we can measure with patterns!”
The first marked instance of the indirect singular they is from some time in the 1300s. Then at the start of the 1800s grammarians of the prescriptivist variety (the kind who think they can make a language do their bidding rather than simply recording what they observe real live humans doing with language) decided that no way could “they” be correct, we must use something else! And came up with “he” as an indirect pronoun.
“If anyone is cold at night, he should put on a blanket.” “Someone’s left his books; I hope he returns for them.”
Thankfully many people ignored that crap so in the nineteenth century we can witness the singular they living on through some authors (such as Jane Austen) and more informal writing while the sticklers did use he.
And then the feminist movement came around in the 1900s, rightfully declared using “he” as the indefinite pronoun to be sexist, and changed it to that clunky “he or she” – “if anyone is cold at night, he or she should put on a blanket.”
But the indefinite singular they lived on and thankfully is regaining widespread use and legitimacy even among the most esteemed grammarians.
And in the meantime the singular they has also been adopted by the trans/nonbinary community, because for goodness sake we need something that frees us from the gendering garbage of this society and when we go for neopronouns like xe/xem or fae/faer people complain those are “"made up words”“ (as if any word sprung fully formed into being without being made up at some point??) so “they” is what many of us select instead. But, shockingly (/sarcasm), transphobes don’t give one shit about all this explanation – they’re going to keep right on denying our pronouns.
Therefore, for another pronoun that works in both the singular and plural, simply direct these people to “you are” – you can even use it in a sentence for them! “You are a transphobe.” “You are transphobes.” Amazing!