Would it be incorrect to write a sentence like “I asked my girlfriend if they would … “, as the sex would already be known due to the use of the word girlfriend? It was my understanding that once sex was known, he or she should be used, not they. I got into a friendly argument about this, where it was brought up that they is epicene, which means it can be used to refer to either sex. I took this as it could refer to either sex if the sex was unknown (“filling in the blank”), but was told that it could actually be used even if the sex was known. I couldn’t find much information on this one way or the other, and am now in a state of confusion regarding this. It’s the first I’ve heard of they being used in such a fashion.
It would be better to be consistent. You could refer to “my partner” as “they,” or you could refer to “my girlfriend” as “she.”
However, it is not unheard of for people to use “they” even if the gender is known. It is slightly less grammatical, but it shows solidarity with people who don’t wish to disclose their gender or don’t consider gender to define their identity.
Language often has to change over time to keep up with societal views. For example, it used to be normal to refer to male performers as “actors” and female performers as “actresses.” Technically, it would be incorrect to call a woman an actor. But nowadays, this happens all the time, and in general there is a tendency to use language that is less gender-specific (because this is more inclusive). You could say that using “they” for a known gender is a deviation from standard English - but you could also say it is evolution.