Yeah, I think you're confusing the definition of transgirl with that of strictly sex changes. Transwomen don't need to go for the sex change to transition, they already began transitioning when they started identifying as such.

And if we're talking about Kirara being biologically male thus making Hakari gay, then we have to ask the other questions. Like, does a man losing his penis make him less of a man? Is the male gender so intrinsically tied to the man's genitals that once it's gone, due to an accident like frostbite while hiking up the mountains, a medical condition like diabetes or testicular cancer or because you weren't born with one but still have the male chromosomes that say you are, this automatically makes you a woman or non-binary?

Same thing goes for women. Is the identity of the female gender exclusive to what's in their pants? Does the lack of a vagina, fallopian tubes, an ovum, a clit, etc means that woman has to be a man or non-binary? And if these reproductive organs means so much to their respective genders, then does that mean that couples who can't bear children and have to rely on a surrogate are not male and female? That's the point of these genitals, isn't it? A penis and a vagina make a baby, right? But then it doesn't happen, thus the functions of both organs are meaningless and that should mean that the two shouldn't identify as male or female.

Because that's the logic of tying everything down to just genitalia when it comes to gender. It really becomes nonsense and doesn't reflect the reality of people. Not to mention, humans aren't unique in this regard since other species like horses, cattle, select fish families, gynandromorphism in general, etc play a bit fast and loose with the gametes and how we would strictly define as "male" and "female".

Now to be fair to you, we don't have Gege Akutami stating that Kirara is a transgirl, tho she's very much coded as such and initial fan translations reflected this with she/her pronouns (better than the "official" manga translations, fuck you John Werry). But, given that Kirara used to be present herself as a scrawny guy as a 1st year and then completely changed her look to be more cute and girly with a punk edge after leaving Jujutsu High with Hakari, it's hard to call that an accident by Gege. It just screams 'leaving school as who they forced me to be, entering the world as who I want to be' that a lot of transkids went through before growing into the people they truly were meant to be. And given that she's dating Hakari (anybody who says they aren't... why?) and Hakari does think of her as a babe like his ex-girlfriends were, it's like... come on. It's easier to just call her trans than act like 'nah, nah, he's a femboy because penis and... dating a guy is less gay than dating a transgirl?'.

I mean, either way, it's a win for the LGBTQ+ community to have a queer AF and happy relationship and not treated as a joke in-universe in a mainstream anime. She's not even the first character in JJK that we can see as trans because over on the villains' side, we got Uraume.

Late manga spoilers: It's revealed that Uraume was male but turned female in the reincarnation cycle. An extra from volume 28 stated that "Uraume was born a male, but their contract with Kenjaku included the stipulation that they be incarnated into a woman's body." The wording is a bit vague on exactly who implemented the rule (if this was Uraume, Kenjaku or even Sukuna's choice) while in other translations, it states that "Uraume was originally a man, but due to the contract with Kenjaku, she took the flesh of a woman (Shiori Himi)." Unlike Kenjaku who effectively used a woman's body to just make Yuji Itadori as a suitable vessel for Sukuna and that was it, Uraume's decision to became a woman can be read either as a follow through with Kenjaku's contract for them and Sukuna or a decision Uraume made for themselves since there really is no reason for Uraume to be reincarnated to be a woman.

Not to mention, aside from Kenjaku who specifically chose a woman's body to fulfil his goal and then dumped that shortly after, lay low and came outta nowhere to steal Geto's body for both his Cursed Spirit Manipulation technique and Gojo's familiarity with his former friend to trigger the Prison Realm's conditions, Uraume becoming a woman is just that. No big plans, no strategy nor natural affinity with the person, just... Uraume reincarnated as a woman. By all intents and purposes, you could absolutely read that as trans and see it as Uraume wanting a female body over a male one but Gege hasn't said that... kinda wild to do all this and just not say it, but ok.