"There was a rather extended period of time in the history of the English language when the choice of a supposedly masculine personal pronoun (him) said nothing about the gender or sex of the referent. It could be masculine, male, neuter, or asexual — and every combination of those three." - Susanne Wagner

It's taken for granted that 'he' is used as a gender-neutral pronoun. I frequently catch myself referring to gender-unspecified animals this way (in place of 'it'). It's been the convention my whole life — exclamations like "whoa, look at him go!" or "that fruit fly must have had a death wish — he dove straight into the vinegar" were common.

Look no further than the English Bible's paternal references to God as He/Him/His for an example of this, even though Christian scholars have long clarified that the being they believe to have created the universe is beyond human conceptions of gender. Terms that are themselves gender-neutral — like 'actor' — are given to men, while women are given the derivative form 'actress.' English will let the gender-neutral form of any word double as the masculine default. In fact, the etymology of the word 'man' is itself gender-neutral.

This is an awkward colloquial situation for some, myself included. Inside of my brain I feel like a human that just happens to have the body that I have — that's it. I feel psychologically androgynous, equal parts masculine and feminine to the degree that they have canceled each other into a neutral space that is secure in its neutrality. When faced with things marketed toward men or women, there's a distance I can feel between myself and the shared experience that the marketing is promising. And from as young as I can remember, I have been proud of the fact that I have a unisex name.

But here's where it gets interesting, and why there is already language for this kind of situation: 'they' as a singular pronoun predates 'you' as a singular pronoun. The introduction of singular 'you' into common vernacular was so dramatic that in 1660, George Fox — the founder of Quakerism — "wrote a whole book labeling anyone who used singular 'you' an idiot or a fool," writes Dennis Baron, Professor of English and Linguistics at the University of Illinois. "Eighteenth-century grammarians like Robert Lowth and Lindley Murray regularly tested students on thou as singular, you as plural, despite the fact that students used singular you when their teachers weren't looking, and teachers used singular you when their students weren't looking. Anyone who said thou and thee was seen as a fool and an idiot, or a Quaker, or at least hopelessly out of date."

And now it’s no big deal. People say "you are" instead of "you is" without any difficulty. Singular 'they' operates with the exact same grammar and can just as well become no big deal.

Part of the magic of the game Undertale and Frisk as the gender-neutral protagonist is the narrative backdrop that they are the only human in this underworld. Their mystique to the characters they encounter has nothing to do with their gender and everything to do with the fact that they are a human. Narratively, Frisk's gender is just that — 'human,' a term they are referred to with throughout the game — as well as child, kid, buddy, pal, punk, brat, dude, and friend, all with they/them/their pronouns.

In public school, my peers would often argue for the gender neutrality of the term 'dude' so that they could get away with calling certain girls dudes. In retrospect, it was a recreation of an old trope — the tomboy, a girl who was "one of the guys." Some of the people who were okay with being referred to this way ended up not identifying as girls later on, because those early signals that they "weren't like other girls" ended up being a little too true for comfort as they continued failing to relate to the shared experiences of girlhood.

Judith Butler put it this way in a 2021 interview:

I suggested more than 30 years ago that people are, consciously or not, citing conventions of gender when they claim to be expressing their own interior reality or even when they say they are creating themselves anew. It seemed to me that none of us totally escape cultural norms.

At the same time, none of us are totally determined by cultural norms. Gender then becomes a negotiation, a struggle, a way of dealing with historical constraints and making new realities. When we are "girled," we are entered into a realm of girldom that has been built up over a long time — a series of conventions, sometimes conflicting, that establish girlness within society. We don't just choose it. And it is not just imposed on us. But that social reality can, and does, change.

If you try to skirt this truth by defining gender with some biological function, like reproduction, then your definition of 'woman' ends up excluding more than just trans women, but also cis women who cannot give birth for one reason or another. Simply having the "correct" genitals isn't enough, because biology is more complicated than that, and nature has always abhorred a binary.

It's not surprising, then, that people — when given the opportunity  — are reaching beyond the colloquial binaries of our language in order to describe themselves in nonbinary ways. English has a gender-neutral pronoun ready to go, and only by fully embracing the social construction of language can we fully embrace the social construction of gender, creating colloquial space for people who feel represented by neutrality.

References:

1. Susanne Wagner via Uni Freiburg — Gender in English pronouns: myth and reality
2. Wikipedia — Gender neutrality in languages with gendered third-person pronouns (section: Reference to males and females; subsection: Generic he)
   1. Patricia T. O'Conner & Stewart Kellerman via The New York Times (via archive.today) — All-Purpose Pronoun
   2. H. W. Fowler — A Dictionary of Modern English Usage
3. Betsy Reed via The Guardian — Not in His name: God is gender-neutral, says archbishop of Canterbury
4. Douglas Harper via Online Etymology Dictionary — Etymology of man
5. Dennis Baron via Oxford English Dictionary — A brief history of singular 'they'
6. Jules Gleeson via The Guardian — Judith Butler: 'We need to rethink the category of woman'
7. Amanda Montañez via Scientific American (via archive.today) — Visualizing Sex as a Spectrum
8. Claire Ainsworth via Scientific America (via archive.today) — Sex Redefined: The Idea of 2 Sexes Is Overly Simplistic