I would say that the British use of mate doesn't really work for any sort of sexual partner tbh. No matter the genders involved that's just not a sentence that would ever feel "correct".
(Which is definitely in stark contrast to the way that a biologist would use the word, but hey.)
Better question: Why do Americans use dude so much when the English version "mate" not only sounds better but is also implicitly gender neutral
to use @kelptea 's test: ask a straight man how many mate's he's slept with...
I would say that the British use of mate doesn't really work for any sort of sexual partner tbh. No matter the genders involved that's just not a sentence that would ever feel "correct".
(Which is definitely in stark contrast to the way that a biologist would use the word, but hey.)