
you have to be in a formal relationship to cheat. also it clearly says that he's in denial that he's loved back. so in his emotional confusion, he goes back to having sex to deal with his trauma. that's not the same as being sleazy. sex is his emotional coping mechanism and his outlet for his trauma.
Also anxiety is very often not logical. that's the point.

the trauma of being gay in a very conservative society, and the trauma of having to keep his sexuality a huge secret from his best friend, and also the trauma of feeling like he had to leave his best friend to escape his stress. He also probably felt like he was betraying his best friend by being in love with him
This whole "they aren't together" argument is very popular, so let's go into that real quick too.
Okay, so we know they have mutual feelings, we know one confessed to the other and when that happened the confessee allowed him to have sex with him without rejecting him (obviously, given he was happy af about it and loves him).
What exactly about that implies the answer wasn't "yes"? At the very least, in Hyesung's eyes it certainly looks like a yes. No arguments against that, if you try you're delusional.
So now, how would this not be cheating, exactly? And even entertaining that it's not cheating, it's still just shitty in general as we all knew upon reading what happened. You can just FEEL it's shitty. We already know his reasoning for sleeping with Hyung, it was "because he's stressed" and "anxious because they're getting closer and he's afraid of losing Hyesung". We have all the facts here. They don't look good...
One final thing, Homin himself says that the reason he's scared is because he and Hyesung are getting closer and the relationship may end, what does "getting closer" reference if not that they're dating now? They were already best friends, what's closer? Boyfriends... He's saying that he's scared of being with him because the relationship may end.