I think what Oshimi is getting to in this manga is an idea he's been attempting over and over and over and over again with almost all of his manga: a rejection of "masculinity".
it's an idea I've grappled with a lot of well, and it's only through reading his other works (Inside Mari, Flowers of Evil) that I was kind of able to flip my perspective. For years and years and years now I've been disgusted by my own sexuality, by wanting women, by jacking off multiple times a day. I had this inner hormonal boy demanding me to have sex have sex have sex, but I really just wanted to be normal and talk to people and treat women like human beings not sex objects. This, compounded with the societal idea that men are disgusting and gross while women are innocent and pure just resulted in a lot of self-hatred. I can't tell you the amount of times I've wished I was asexual or that I could chemically castrate myself. This feeling, this frustration and internal hatred, is what is being portrayed through Yo.
At the same time with all of this self disgust, women are glorified. Both as objects of attraction in a perverted mind, but also as innocent and near perfect beings who dont have to DEAL with this perversion. Mix all of that together and you can't help but wonder, do I want to be a girl? It's not that I hate being a man, it's that I hate men in general, I've been told time and time again that what I am is disgusting, perverted, and dangerous. What if I could be the other? Attractive, cute, innocent?
I think this is part of why Kei seems so "pure" to Yo as well. Within Kei rests the idea of not being loved by a women as man (as an object of sexual performance), but the opportunity of being seen the same way he views woman. To Kei, he isn't a beast of sexual performance, he doesn't have to put on a show and be a man, he just has to be himself. It's as if society falls away, and all the pressure that's been put on him his entire life falls away with it.
I kind of want my own Kei.
In the end, Oshimi is just doing what he does best, taking struggles with sexuality and gender, and pushing them to the extremes so we can see how they really work.
