Kousuke is letting the adult guilt get to him. It's the old "the younger boyfriend would be better off without me/I'm leading him astray/he's missing out on all the normal stuff because of me" trope. In his case the guilt is being compounded by his friends who aren't supportive and feel like he's dragging Setti along instead of it being a proper mutual relationship. This infuriating trope is usually followed by a break up, miserableness for both parties, and ultimately coming to terms with the fact that the break up was a mistake and getting back together. I hope the author will change things up and Setti won't give up and hang on, but I don't know if he can muster that much courage in his current state.
What the hell is happening here? Kousuke?!?