Non-binary option for characters, please?

[Iscah]

I think it's less a matter of it being "polite", more a matter of it having a second meaning that is considered taboo.

That's what I meant.

Interestingly it doesn't seem to have that definition in the 1964 dictionary - and not because they didn't want to talk about it, it's just a separate entry.

But we can fix that very easily.
"Mr. Jack's gender at birth was that of a woman. His gender mentally was always that of a man though."
And voila. Problem solved, except that it still looks so-so with the word "gender" repeated in consecutive phrases. However since the word "mentally" was added, the issue of misconception is solved because now not only first sentence have context (at birth), but the second one have as well (mentally). Something the phrases above lacked.

I still don't think that's correct - or ideal at any rate. That's exactly when you use "sex" and "gender" as separate terms with different meanings, even if they can be interchangeable elsewhere.

[kikix12]

I still don't think that's correct - or ideal at any rate. That's exactly when you use "sex" and "gender" as separate terms with different meanings, even if they can be interchangeable elsewhere.

Its meaning is correct. Synonyms have that to them, that they are not selective.

However it is a stylistic error, at the very least. Under normal circumstances, any person with a suitable vocabulary should avoid making such phrases. Completely non-problematic alternatives are, for example:
"Mr. Jack's gender biologically was that of a woman, but mentally he always was a man."
"Mr. Jack's gender is biologically a woman and mentally a man."
"Mr. Jack's biological gender of a woman is different than the mental one."

It's all a matter of context. When using a multi-meaning word in an ambiguous manner, it is a good idea to provide context. There is no such need when using two words that are synonyms in one of the cases, but not the other, like "sex" for biological gender and simply "gender" for mental one. That is part of the reason why we use synonyms.