Is he with the guy in the past or the present. If it's the present, that's just having a boyfriend, right?
If it's the past, well, Hamura didn't think of them as "going out" and didn't admit he loved Jin, so it's hardly cheating either, if what he considered them were sex friends.
From what I've got, Jin and Hamura did go out Chromiee
Chapter 3 doesn't make it seem like he thought of it as love or going out. Just friendship with some playtime. (In denial, I assume). You never hear the words one expects when going out/dating/in love.
I've read the raws and I still don;t understand shit besides Alex being a hotchocolatayaya boi. Did Jin cheat on Hamura with Alex?! WTF IS GOING ON!