One of the reasons why I think Rei's euphoria ending haircut doesn't actually piss me off all that much is because it reads to me like Rei going through his maximizing-masculinity era of early transitioning.
What I think a lot of people miss about Rei's gender-questioning in his route of the story is the fact that Rei does clearly want to be a man and feels most comfortable in that identity, but also feels like he isn't allowed to present himself that way due to his shitty fucking dad telling him he could never be a Real Man so long as he's gay. This isn't to say that Rei is completely uninterested in ever exploring femininity either, clearly his current appearance shows that he's at least comfortable enough with it to be willing to experiment somewhat and stick with a more feminine look/personal pronouns, but where he gets caught is in feeling like he isn't able to do the same thing with masculinity, other than through pure violence and fighting.
We get a kind of symbolic representation of this through Rei and Towa's first attempt at Euphoria-- Rei is desperate to literally cut off his dick, to fully subsume himself into a New Identity that he feels like is the only one he's allowed to have, only to back out at the last minute. He can't fully part with this Symbol of His Masculinity because, on some level, it's still what he wants. What he lacks is the self confidence to redefine that masculinity for himself, to find his own meaning of what it means to be a man separate from the standards set and taught to him by his father.
Similar to how Rei enjoys doing body art on other people but never can quite do it to himself, Rei appreciates and is attracted to masculinity in other people but doesn't feel like he is allowed to embrace that identity in himself, struggling to come to terms with what feels like is a contradiction between his gender and sexuality. As his father taught him, women like men and men like women-- so if he's going to be attracted to men, he is no longer justified in wanting to be one himself. To anyone with even a slight acceptance of the Concept of Gayness this naturally seems absurd, but to someone deeply entrenched in the worst muck of heteronormativity, I think this idea is actually a shockingly easy to stumble onto.
To speculate a bit, while it is never super explicitly stated in-text, I do think it's also important that Rei's main friend group of Honami, Junko, and Arata are so transfem/nb-coded. I think it's easy to see how those four could've possibly grown closer through similar experiences of growing up with all the expectations of being a Boy, only to shift into something more feminine later in life. But, similar to how the rest of Rei's route is about him being unable to fully trust or confide in his friends despite their closeness and his love for them, in Rei's case his situation is more complicated, and that seemingly (perhaps, only to him) isolates him from the rest of them. Again, Rei doesn't actually want to be anything other than a man, he doesn't explore his attachment to femininity out of a genuine desire to present himself that way all the time-- rather, he is forcibly pushed away from masculinity and ends up where he does in reaction to that.
In a way, you could almost read Rei's route as essentially a cis guy going through a similar process as a trans guy, getting forced into a more feminine role by his father's homophobia and then eventually making his way back out to an identity he actually wants and is comfortable in. Really, I think his route works perfectly as a way of exploring how even someone who is, at the end of the day, basically cis can still go through a period of questioning and confusion and dysphoria, even if his end conclusion is that he wants to stick with his original label. It's the process of understanding and defining that label for oneself that really matters here.
(Personally, this is why it's so important to me that Rei is recognized as a man, because it is this exact kind of gender questioning that I resonate with. There was an extended period of time where I myself wondered if I might be a different gender identity largely because I hated how I was treated as a girl growing up (#misogyny), but it was only through hearing from and interacting with more trans people in general and women in particular that I began to figure my shit out. This is part of why I think a term like "detransitioning," does so much more harm than good, even when used in good faith-- you should not and will never truly be locked into one singular strict identity, and the process of figuring yourself out is a net positive in my eyes regardless of the conclusion. But, anyways...)
Furthermore, like the rest of Rei's route, there are also interesting parallels to Towa in this regard too, especially if we take into consideration Towa's extremely negative reaction to being forced to crossdress in Taku's route. Notably though, I think Rei and Towa's specific traumas result in very different relationships with gender and masculinity. Where Rei feels locked out of masculinity, like being a man is something he simply isn't allowed to be so long as he continues to be attracted to other men, I interpret Towa as instead getting turned off of femininity due to its deep associations with his own (forgotten) traumas.
Towa associates femininity with his mother, with the death of Mei, and thus attaches womanhood to his childhood of broken trust and parental abuse and grief. While Towa is also just gay, I think you could also read him as being on some level freaked out by women as they resonate too heavily with the Things He Wants to Forget, finding comfort in masculinity and its associated violence as a more subtler way of self harming without having to look Directly at the things that hurt him the worst. It's complex, of course, and there is still something to be said about the ways in which Towa recreates some of his past experiences with Mei with Lisa in the Taku route, but I think a lot of that can be mixed up in the kind of partial-healing that we see Towa undergo in the Taku and Rei routes in particular. Towa never really seems to question his masculinity in the same way that Rei does, more easily accepting that he can both like men and be one, but that doesn't mean he doesn't have his hangups either.
Anyways. To reiterate my first point, this is all to say that this is why Rei's super short haircut makes sense to me. It fits in my mind that this might be the period where Rei is first starting to experiment with masculinity and embrace it in a way that he wasn't able to before, immediately gravitating towards the things he understands as more Typically Masculine and trying to hold himself to that standard after feeling like he wasn't able to in the past. It seems perfectly natural to me that there's a Rei, further in the future, that may feel more comfortable embracing some of the femininity that he did previously, once he gets more settled and comfortable with himself. It's a process. Let him go through the give and take for a while.
