I knew this ask was coming lol.
I generally refuse to answer questions about Mutsuki’s gender because the discourse is bloody terrifying. But since the plot appears to be addressing it, I’ve spent the past two days thinking about it, and I’m going to answer this but under a cut.
Please stay out of my inbox as I really don’t want to engage in discourse beyond stating what I’ll say below.
Let’s discuss real world vs. fiction. In the real world, gender is not a binary. Gender is a social construct usually based on whether you’re born with a penis or a vagina and I firmly believe this (guess what people, some countries recognize other genders beyond just male and female and some always have, plus intersex people exist and it’s a lot more common than you’d think). Gender is whatever someone says it is. Thus, according to the real world, if Mutsuki says he is a boy, he is a boy. End of story.
For that matter, plenty of people who are transgender have posted about how they can relate to Mutsuki and how his words are not inconsistent with how transgender people feel. Their opinions should be respected. As a person who identifies as a cisgender female and who is not Japanese and therefore does not understand the context, I do not feel qualified to offer my opinion and say “Mutsuki is definitively x.” And I certainly don’t feel qualified to tell other people how to refer to Mutsuki, or how to think about him, because the text has not been clear.
That being said, my personal opinion is that the narrative is presenting Mutsuki as a female disguising himself as a male for protection, as @dreamofcentipedes says here. I’ve thought this since chapter 13 just like DoC says. My personal opinion also is that I think it’s a form of queerbaiting, if that is indeed the direction the story is going. It does not affect my love for the character though, and guess what, there are people IRL who are confused about their gender too, and their stories deserve to be told. I know someone who was assigned male at birth, presented as female for three years, and then decided to present as male again. But the implications of Mutsuki’s gender issues in a real world context do make me uncomfortable. Yet also remember I’m reading through a western lens so take this with a grain of salt.
Remember when I said in my incoherent reactions post “Oh look a Mutsuki flashback like I’ve been predicting… I’m dreading what it says in the text because I can wager a guess (no my dread is not Mutsuki-fate related)”? Yeah, this was exactly what I was alluding to. I’m tired of the lack of representation in media.
I still think the story is not 100% clear in its writing so we’ll probably get more clarification (like a decision). See, when Mutsuki is sanest, he refers to himself as male and it seems that he identifies his female side with his violence. There’s also his relationship with Urie and how Urie having one love interest who presents as a man and being the object of desire by a gay man (Matsuri) really ties in with Urie’s arc of rejecting the heteronormative toxic masculinity that’s killing him. But this could just be unclear writing because no story is perfect.
However, as you say, yes, 155 does imply the same thing that other chapters have implied: that Mutsuki sees his presenting as a man as a lie. Which I find icky personally. Though it’s also worth noting he’s out of his mind right now so we’ll see what he says when he’s brought back to himself and I’m not making any snap decisions until that happens (though in ch 13, he was in his right mind). But that does appear to quite possibly be the narrative implication and I’ve thought that for a good long while. There’s also that, as I’ve said, I’m not Japanese and it’s entirely possible that we’re not supposed to interpret him as a transgender character–I have heard that the Japanese fandom does not consider Mutsuki transgender and really never has, so our western lenses might be mixing us up. If Mutsuki does end up deciding to be male, I’d be pleasantly surprised, but ultimately I just want Mucchy to be happy because I love him.
I personally will still refer to Mutsuki by masculine pronouns unless he has a moment like Karren did, while he’s in his right mind, like Karren asked to be called by her female name and referred to it as “dying as herself.” (Though some fans still didn’t find this clear, and I understand and respect their preference to use “they/them” or even “he/him” for Karren.) Given the real world implications, I prefer to just call Mutsuki what he wants to be called because transgender fans have multiple times expressed that doing otherwise hurts them. But I do understand why people use feminine/neutral pronouns and don’t think it’s necessarily morally wrong for people who do so, because narratively, that’s entirely likely to be where the story is going, and it’s fiction, not real life.
But, I’m still going to call out Mangastream for their ridiculous mistranslations, where in they use female pronouns for Mutsuki when the Japanese does not use pronouns and they totally can translate it without using pronouns. Like, they even have Takeomi use female pronouns when Takeomi almost certainly does not even know Mutsuki was born with a vagina. That kind of mistranslation is a blatant disrespect to his character. Like I’ve said, I’m not telling people how to refer to him outside of the story, but using female pronouns within the actual translated story when his gender is likely going to be part of his character arc and always has been set up to be–whichever direction it ultimately ends up going–is really jumping the gun and setting MS up for an “oops” regardless of whether Mutsuki decides to be female or male, because it’s probably going to be part of his character development. Like Mutsuki’s gender was brought up in chapter two of :re. How could you not think it was going to be important later on? It also implies they care more about their personal interpretations than they do about the actual text plus is insensitive to the real world. Anyways, dear Mangastream, I am sincerely grateful for the translations, but also in regards to this?

(this absolutely doesn’t apply to fans who use female pronouns, as I’ve said, I lean towards their interpretation of the character, though fans who try to force other fans to refer to Mutsuki with female pronouns–cough reddit cough–can, in fact, also take this to apply to them)