It's so sweet to follow their progress through life as they grow older (and as I grew older myself lol), the hardships they face and the good and bad times. The gender identity and discrimination still plays a great role and it's great to see how the two of them adapt to living together and balance responsibilities and stuff. There was this one moment that the author put in which was really impactful and gave me a lot of feels, which happens about ten years or so after Mamiya became a female forever, which is when Miyoshi remembers male Mamiya and realizes he will never see him again and it is a very emotional moment, but then he realizes the female Mamiya in front of him is the same person and is so happy to have her in his life ;__; that was a very beautiful moment and the author portrayed it really well and even tho I finished this a while back that scene still really stands out in my memory and it is one of the things I remember most from this manga ;___; so many feels!!!

Okay, so I'm happy and all that Mamiya didn't suddenly become a 'girl', but I found it weird that she was okay with wanting to change her gender to a girl so casually just because his sex changed too. Or at least it seemed that way. Even now I have to think a bit whether to refer to Mamiya as a girl or boy when I type this, because at first it seemed that the only thing that seemed 'girly' that Mamiya changed in his lifestyle after was wearing girl's clothing. That's it. So did he change his gender too? Or since it isn't mentioned, does that mean he doesn't identify? Or are we supposed to assume that his sex and gender change altogether? I know this is a kinda complex subject about gender identity and sex, I myself am not that well-informed either, but it does bother me.

So when Miyoshi claims that he would still love Mamiya even if her sex changed back, it did seem somewhat believable and he even scolds himself for loving Mamiya only after she changed.
Then that's where the ending kinda ruined it a bit for me. AH OKAY so I admit my bias towards BL is showing. But in my opinion, I feel that the story would've wrapped so nicely if Mamiya did turn back into a male. She went through that cocoon thing just to rework herself into a female again, so that meant she did return to a male, but decided she'd leave him for 5 years in doing so.. That part was so sudden and caught me off guard that it felt really rushed and irked me. If they had fleshed that skip out some more, and didn't just have it get tacked on at the end, I think it could have worked, despite my bias. But yeah, Mamiya even said she'd have emerged from the cocoon earlier had Miyoshi said it was okay either way, and it's like, THAT COULD HAVE SAVED YOU GUYS FIVE YEARS. Or however long she was in there for. I think I would've been more okay with that if maybe Mamiya's thoughts were revealed? Like, if she was worried that society would accept them 'more' if she was a girl and that's why she felt she needed the cocoon.. But the whole, it being two guys originally issue wasn't stressed much at all (which I thought was nice) so I don't think Mamiya would have cared so much, and now I'm thinking was it really necessary then for that whole cocoon ending other than needing to keep her as a girl and to have a little drama last minute?

Sorry I typed too much, but I really did sort of like this. I just think it would've had a powerful message (and, admittedly, satisfied my BL bias) had Mamiya's body reverted back to a male's and Miyoshi did continue to love him despite only having feelings for Mamiya after his body was a girl's. I think it could have been touching if Mamiya was more conflicted about keeping her body female because Miyoshi only started loving her because of that, and then Miyoshi stopping her from cocooning to reassure her that his love greater than that. ALSO, maybe it could've worked too if the side effect about reverting sex back wasn't there at all. I don't know.

Anyways, these are just what I feel, sorry about my BL bias (I tried hard to be more open minded okay! I will just let all this be my headcanon xP), but I still think things could have worked with Mamiya being either sex had the ending been reworked. I like to think that gender/sex shouldn't matter in this love story so much, except for the fact that that's a major plot device so it has to be an issue xP

The way it is presented and explained towards the end is that it is not that person randomly emerges, but rather they emerge so that they can have children with a person who is dear to them.

For instance, Mamiya stated that he needed to adapt to his new body (indicating that he still identifies as a male) + Miyoshi was okay with Mamiya reverting back to his original body so I don't understand the need for the first half of the story. If they realized that they were in love with each other regardless of gender then it was pointless for Mamiya to spend years in the hospital and risk his health by remaining a woman. Especially since Mamiya didn't quite enjoy all the changes anyways. It just seemed pretty heteronormative for the story to head in that direction.
Also, some of the statements in this manga were really sexist. I'm sure the author didn't mean any harm since a majority of Japanese people (any maybe the entire world) are brought up to believe in a binary system with specific gender roles. It was just too obvious to ignore, and it pops up throughout the entire manga. There was a point where Mamiya states "when I was male, no matter how much I loved you I couldn't achieve this feeling" which he believed was due to him becoming more feminine? It just makes no sense to me that you could love the same person more by: 1. changing your gender and 2. being feminine.