Cheating is Diesel fault. People who cheating can't make exuses like: I was confused or this happened so quick. People who cheated also oft are naive in love, not know them self and don't know how to love. Probably most people are confused when they cheating, but it's not justify them.
Diesel has own will and just cheated his lover, that's lame.
He can not lying Yanek about his feelings towards him and sayig how much important he is for him., then going to his friend and sleaps with him.
He can first end hi realtionship with Yanek and sleap with Ein.
What Diesel doing is cheating and wrong.
Most people, who cheted have dilemma, but it is not exuse for cheating.

I like Diesel very much. I think Diesel is naive in love , he doesn' t know himself , he doesn' t know how to love. Even if he lived with Ein for a long time , he' s not sure.
In chapter 53 , Diesel came to Ein to warn and fight , he did it for Yanek. However , the Ein' s action is beyond the forecast , it makes diesel awake inside him. I think it' s not Diesel' s fault