I mean i could see it, but as the story is displayed if this is the case, i think its portrayed poorly.
I mean to me that was the take away from the whole past couple chapters. This whole thing has been about them exploring their identities and coming to a conclusion. Kei's transition to a woman just wasn't their conclusion, but merely a part of their journey.
I do get the frustration of "hur hur being trans is just a phase" though, but that is simply just some people's reality. Sometimes you just don't know who you are until you try and fall out of stuff. Not that I want to discredit trans youth, just saying your mileage may vary. Some people know for sure with whole confidence right off the bat, and others just really don't have a damn clue and jump around with their identity before finding the right fit. (but maybe I'm just projecting my own queer experiences here, I didn't attain Q status until my mid 20s bc I just had no idea, and hell sometimes I'm still questioning.)
Now I'll preface this with the fact I've been reading this as it's dropped and haven't had a full re-read (ergo older chapters may be fuzzy to me) but my interpretation of Kei has kinda been something like:
Start: "I don't feel like a boy. I don't like being a boy. My worldview has only ever shown me a binary though, so since I'm not a boy I must be a woman"
Mid: "I'm happier as a woman, but I still don't feel right. I'm frustrated bc this isn't all I thought it would be. I still feel incomplete. What else is there though?"
These past couple chapters: "I am now finding out what else there is
by having gay sex with my bestie by shared experience with another gender confused friend. I don't need to be either a man or a woman to be loved and to be human and have human experiences. I don't need to be a man or a woman, I can just be."
Or I'm just reading too much into Oshimi-Sensei's fetishes idk. Thank you for coming to my TED Talk lol
edit: after reading that afterword, I think I'm pretty spot on lol.