Kirky finds it strange that a plural of mixed gender should always take the masculine form.

Well, that's been one of the whines of the feminist movement. It is a fact that in every Indo-European language I know anything about that when you have a mixed plural or a word with an indeterminate gender, the default form is the masculine. It gets more strange in the Romance languages without the neuter gender. In French, it is always "he" who rains or beams sunnily, as in "Il pleut, il fait beau." Same in Italian, but since the pronoun is usually omitted, it's not obvious.

The feminists' wrath against the use of "he" as the default for the indeterminate pronoun is the cause of the new use of the plural pronoun instead. There are plenty of us old-timers who hate this usage although having some (not much) sympathy with the ladies. I do not like a usage like "If anyone wants a cup of coffee, they will have to go to the kitchen for it." But this has become so common that now it's getting to be a mindless reflex and you have outrages like "If any new mother wants home nurse visits, they will have to call the Home Service Dept." Or, "If any father wants to sign up as a coach, they need to see Joe Smith." I have actually, and more than once, heard such a sentence used by newscasters. It has got to the point where this neologistic plural is used for all purposes, even where there is only one gender involved.