Actually, there are dozens of gender-neutral pronouns, and that’s  true even if you limit your search to the science fiction canon. But calls for a gender-neutral pronoun are much older than you might have thought, as the Oxford University Press blog explains, and we still haven’t managed to adopt one [via TheBigThink]:

Such discussions in the 1880s and 90s did nothing to shake up the pronoun paradigm, and nothing came of subsequent proposals for heer, hie, ha, hesh, thir, she (together with shis and shim), himorher, se, heesh, hse, kin, ve, ta, tey, fm, z, ze, shem, se, j/e, jee, ey, ho, po, ae, et, heshe, hann, herm, ala, de, ghach, han, he, mef, ws, and ze [a list with dates and sources for many of these pronouns can be found here].

Flash forward to 1978, when The Times (of London) prints a letter in response to yet another call for a new “unisex” pronoun set, advocating le, lim, ler, and lers. (And another correspondent tersely suggests it.)

Despite this wealth of coinage, there is still no widely-accepted gender-neutral pronoun. In part, that’s because pronoun systems are slow to change, and when change comes, it is typically natural rather than engineered.

For those of us who work with words, of course, there are canonical rulesets to which we are supposed to adhere. But it’s the ruleset of grammar that long forbade the use of the singular they:

… despite the almost universal condemnation of the coordinate he or she by supporters of gender-neutral pronouns, the rule books now opt for he or she and not an invented word to replace the generic he. Students who once were taught that the masculine pronoun must always be used in cases of mixed or doubtful gender are now taught instead to use coordinate forms, not for gender balance or grammatical precision, but simply because that’s the new rule. Those writers who question the rule, who realize that multiple he-or-she’s just don’t make for readable prose, won’t seek out a new gender-neutral pronoun. Instead they’ll recast some sentences as plural, and for the rest they’ll just take their chances with singular they. After all, if you, which is also gender neutral, can serve both for singular and plural, why can’t they do the same? In any case, after more than 100 attempts to coin a gender-neutral pronoun over the course of more than 150 years, thon and its competitors will remain what they always have been, the words that failed.

Regular readers may have noticed that I tend to use the singular they wherever possible – indeed, I’ve been called out on it in the comments here once or twice, so that grammatical rule dies hard. I really can’t remember when I started doing it, either; I’m not sure whether I was taught that way at school (though I doubt it, given the conservatism of my education).

All this, I suppose, makes gender-neutral pronouns a case study in the seemingly universal human urge to create multiple new rules in order to fix a problem that could be obviated by dropping or loosening a single old rule…