So, I decided I wanted to further elaborate on some of my feelings about what's going on at this point in the story. Partly because I feel like it, and partly because I really have nothing better to do.
Personally, I think Shuuma ought to give Koshimizu a shot... well, more of a shot than he has already. She seems very sweet and caring, and she's made it pretty clear that she's willing to indulge his fetish. Although, speaking as someone who has this fetish, I can tell you that doing it someone who's merely okay with it isn't quite the same as doing it with someone who's also as into as you are, so I can understand that. And that's to say nothing of the fact that the alternative is his teacher, and realistically no good will come of that.
All that said, and as much as I feel sorry for Koshimizu, one thing is important to remember; Shuuma does not owe her a relationship, and if he doesn't want to be with her, he doesn't want to be with her.