To me, it feels like the whole reason for Susa's reaction - i.e. "I'm going to end this right now" - stems from realizing Amane was into watching Miko & Saku going at it, while Susa himself was barred from that via blindfold. Funny how while being watched and able to see Miko & Saku himself, everything was fine, but the moment he doesn't get to have the visual aspect of it there's suddenly a problem.