The Far, Bright Center

I enjoy headcanoning both Kirk and Spock as nonbinary in the same "technically but badly closeted" way that I interpret them as very much bi4gay—but also, as pretty different types of nonbinary. Spock describes himself as both a man and not a man (within the same episode, the iconic "Amok Time"). In many respects, he possesses the most purely unassailable masculinity of anyone in the show (this is a significant plot point in one of the Maximally Gender episodes, "Charlie X"). He wears make-up in the style of female Vulcans like T'Pring and T'Pau more than like how most men wear make-up, while surrounded by people who don't even know the difference, and at the same time, he's not at all uncomfortable with being identified as a man. To describe Spock as a man is incomplete information, not false. His "nonbinariness" consists very specifically of Man and Not-Man, and he tends to be marked by a highly consistent presentation of himself that blends gendered conventions, as suits his unique experience, but also makes him the most supremely masculine figure around when that's an issue at hand—a specific sort of bigender quality with some pretty obvious resonance with his experience as a biracial Vulcan.

Kirk, I think, doesn't consciously consider that he could be anything but a man, and is broadly okay with that, if the range of his gender performance isn't compromised by external pressures. But when he is pressured to occupy some specific gendered role, his resistance seems to go from 0 to 100 very fast. I imagine nb!Kirk as the kind of closeted-even-to-himself nonbinary person who assumes everyone's experience of gender is as tied to performance as his own, and surely, it's just obvious that it's all kind of fake outside of social dynamics, it's just that social dynamics affect people's lives and psychologies, and thus matter (the reality is more complicated, obviously, but it is a not-uncommon perspective among some kinds of nb people still figuring our shit out, cf. Judith Butler—my headcanon is that he's less bigender or multigender than "agender diva who likes to fuck around with conventions around masculinity and femininity and whatever else, but feels little allegiance to any of them as a stable state of being"). I do think that being reduced to a specific, exclusively masculine role by forces outside himself (despite being sometimes useful) pretty evidently grates on him more than the somewhat effeminate roles he sometimes gets steamrollered into (also sometimes useful), but since he's strongly implied to be AMAB, that wouldn't necessarily be unusual (assigned gender often has more baggage for obvious reasons, even if it's not more or less "wrong").

I do tend to think of him as transfem-leaning nonbinary at heart (one of my many quibbles with Tumblr TOS fanon is that I genuinely think Kirk makes 100x more sense as transfem than transmasc, and that his presentation in the peak Kirk Enrichment Enclosure episodes is far closer to femme than butch too when accounting for the limitations of the era), but the cisheteronormativity of the 23rd century is alive and well. The specifics of how he would fully express his actual sense of gender in a less restrictive world don't preoccupy him much as long as he gets to be the diva he was born to be and doesn't feel (gender-wise) like someone is actively clipping his wings. So he's just sometimes going to slip into announcing that gender is an insignificant distraction from the common personhood of all people, if a fun one, before breezing onto picking flowers or throwing himself into the occasional community theatre production or fluttering his eyelashes to escape the third trap of that week.