
Honestly from what I see Diesel romanticlly liked eni but not yanek. Also yanek and diesel never defined their relationship so they could have just been "sex friends".

Exactly ! I don't think they're lovers too.
Not even sex '' friends. ''
I mean Yanek saved him and hepls him when he's in heat. And Diesel, relieves him sexually, and at the same time kills people in his stead.
Though Yanek did help him mentally, I think that's all between them.
And for Ein, they look at each other which so much hidden feelings I can't stand how in love they seem to be.

Look on this. Diesel and Ein never confirm their relationship. They live together sleeeps together, have some feeling towards other person , they are tender for each other. They are in relationship, cause how they live, not cause some big confession. This same is with Yanek and Ein.
In adult world You can be in relationship, even without big words.
If Diesel goes and sleeps with Bexan, the he would cheating Ein, this same heppens now.
Diesel goes to Ein and sleeps with him, that's why he cheating Yanek.

Honestly sleeping together doesnt mean dating or in a relationship. Even in the "adult world" a relationship will have to be defined before you can say "that's cheating". And I'm gonna say the same for diesel and Ein in the future too. Diesel doesnt know what love is which also doesnt help him. Maybe he'll learn about healthy relationships(dating etc) and how to love in the future but it sure wasn't and isn't gonna be with yanek.

For me is litlle naive that people think that only big words or confession must be to be in relationship.
Are some pair, who.never does it amd yet still live in relationship many years.
They sleap together, living together, don't have other partners, they have feelings towarfs other ( not Diesel he laying about this :/ ). They are in relationship.
Diesel chated him. If he dosn't want to be with Yanek he should end this relation first, not cheating him.
He doesn't necessarily have to be bi. He could be pansexual or demisexual.