@Dramsss hmmm, lots of interesting avenues
1. i don't think it's him not being able to control himself, he totally is not in a position to feel this kind of thing. he's hyper stressed. every time he was in such a position thus far, it's been Koto that went onto him.
2. kissing her to break free: maybe, but in such a place, he would do that there? he never was forward with her. i dunno.
someone mentioned that in that lower right panel, the hand that i also thought was Kousuke's is in fact Koto's. she's very relaxed while kissing him, she gently lifts her hair, that's what's bugged me from the start and stopped me from thinking he's forcing her. the idea of a set up feels quite possible to me.
if it is indeed Kousuke that initiated the kiss, he totally doesn't like it, his face keeps looking forced, while Koto's look relaxed, even though it's probably her that asks to stop, but it's either not a true "stop", you know, like a "i like it stop but don't," or she changes her mind at that point (which would make sense for a set up).
one thing i thought about that i haven't read anyone mention is that, MAYBE, MAYBE, maybe he might've picked up on her "i like when it's the full moon" cue, that AFAIK, has a meaning linked to expressing your love for Japanese people, "tsuki ga kirei desu ne", maybe she was hoping he'd reply to her clearly, or maybe she was baiting him there (would point to a set up), so maybe that's why, when she pulled on the string and got closer to him, he thought well fu*k it she wants me to kiss her i'll do it.