Responses
They didn't have mutual feelings yet, but they were already in a relationship per se. They promised to be exclusive, but Mizuki couldn't hold out, so it is cheating. I hate it too, but can understand the circumstances. Besides, I do believe Mizuki when he says that he only attempted to break the promise, which suggests that he didn't get to feed/have sex with that dude.
I HATE cheating but I guess in this case they weren't official because they weren't clear with their feelings and for the uke it's like being starved without food