This has become a very interesting discussion.
^Which is another reason it doesn't fit, because in British/Australian usage it just means "friend," not "committed romantic/sexual partner."
True, true. Being British myself, I have indeed used it in that context before (though not as often as most of my friends, probably because I was more comfortable with the word as implying a deeper relationship, at least at first). It might seem silly, that I have two different senses of the word, but on the whole I have no problem with the dual usage; to me "mate" just has different impact depending on the context it's used in. Like the word "friend", I suppose; calling a casual acquaintance or a stranger "friend" is very different from using the word with a true companion, though I've no doubt the former situation borrows from the latter, suggesting a somewhat exaggerated sense of goodwill when you appropriate "friend" to use on a stranger. Perhaps the latter usage is the more "correct" one, but in that case surely it's the use of "mate" to mean friend in British/Australian vernacular that's the "corrupted" use? I sort of assumed the same sort of thing is happening there as happens with "friend" - when a friend calls me "mate", they're borrowing the shell of something more powerful than is suited to the content they're currently wrapping it in, for effect if nothing else. Sort of a linguistic false advertising, I guess, wrapping a mild feeling in the packaging that usually contains a strong one, to bypass any awkwardness and encourage acceptance of the bond between you. It's something different behind the word but paradoxically it isn't? A least that's been my assumption. It seems to me that the widespread acceptance of "mate" in the common casual use is a reflection on the emotional power of the word in its "true" use. So I might as well acknowledge the power of the original.
Oh, I'm gabbling again

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To me, "mate" sounds almost demeaning -- like you're talking about an animal rather than a person.
Interesting. To me, it speaks to the depth of the relationship - perhaps because it
does suggest something primal; that, rather than a relationship defined only by changeable social conventions or current whims it points to a bond that's rooted in the most instinctive behaviours, enhanced by our sapience. A connection that resonates on every significant level of our beings. In all, I suppose I'm just one of those people who doesn't shy away from our "natural" heritage; I don't necessarily see "animalistic" connotations as degrading.
Also, for what it's worth, "mate" doesn't carry any "breeder-centric" connotations to me either (though I can see why logically it might), but of course that's sort of the point - I have my own sense of what it means (and I guess I have a habit (bad habit?) of finding my own way around the language).
I've noted it's fairly common in Sci-fi, including Trek, to use "mate" for a long-term or serious partner; I find that makes some degree of sense. After all, whatever cultural systems a given race has governing initimate relationships, all those races are responding to the same underlying directives, and I think the term "mate" acknowledges that, speaks to a common ground the majority of species would share.