I think that the resistance to use “gender” to mean actual biological gender is a regional thing, not really based on a proper usage.
To me, “sex” refers to the act of procreation and related concepts. To use “sex” to refer to male or female is uncomfortable and awkward. It’s as bad as using that “p” word to refer to a kitty cat—which, in some dialects, is perfectly okay, but not in my dialect.
And the use of gender in language, in those languages that have it—people really do think of some objects as feminine, with feminine qualities, and others as masculine, with masculine qualities.
I think you are wrong. When I took biology, the word gender was never used. I heard it a lot in Spanish class, though.