Egalitarianism and anti-sexism seem to be strict norms in English-speaking societies. I think a majority of people advocate the use of they when referring to a specific definite person whose gender is unknown to the speaker--as shown, for example, in the comments to the following youtube video:
For example, such people think the sentence Leslie did their best is acceptable, where Leslie is the name of a person whose gender is unknown by the speaker.
When gender is irrelevant or unimportant, referring to a specific definite person as he(she) when the person's gender is known by the speaker may be considered as sexist by those advocates of egalitarianism and anti-sexism.
So my question to ELL is this:
Is referring to a specific definite person whose gender is known by the speaker as they acceptable?
For example, is the sentence
The police officer John Smith did their best.
acceptable when the officer's gender is known to the speaker?
Remark Since this question has several downvotes, I would like to make it clear that my question is ABSOLUTELY NEUTRAL. It's just a question. No more or no less.