>No, no it is not. I assure you, it is not correct.

Your assurances don't mean much compared to usage going back to at least 1382: https://en.wikipedia.org/wiki/Singular_they

It is correct. Usage makes it correct, and it has been used this way for centuries. Its your incorrect labelling which results in the unnecessary spread of gendered pronouns.

Despite your assurances, the writers at the OED and Fowler's seem to disagree.

https://public.oed.com/blog/a-brief-history-of-singular-they...

>No, no it is not. I assure you, it is not correct.

Language evolves, and this means that the rules of grammar do too. The MLA already allows for pronoun usage to match that of the preferred pronoun of whomever you are writing about, and are considering allowing for the singular they in general. Some style guidelines already allow it. Others still proscribe it.

Plenty of news organizations, including the Washington Post, have set their style guidelines to use the singular they.

It's grammatically correct in plenty of places today, and the current trend is for that to increase, not decrease. You're fighting a losing battle here.

Another example of language evolving: why isn't it "I assure thee, it is not correct" when addressing a single person?

Well, everyone is entitled to their own opinion.

Yes, and some opinions happen to be plain wrong, and others are similarly entitled to say so. The entitlement to state an opinion does not guarantee its correctness.

Ok, I guess that joke was a little bit too subtle.

Let me rephrase that reply: Everyone is entitled to his own opinion.

Everyone takes the singular, but in that reply, I paired it with the plural their.

Yes, and everyone missed the joke because that sentence reads naturally, thus proving that "their" is an appropriate pronoun to use.