Sure, I'm not a fan of this webtoon, but still I'm up to date with chapters ;)
<Spoilers> well, taking into account that Naoru was a prostitute, and left the business by the end of season 1 as I remember, then yes, there was 'cheating' in some way, but once he met Hiroshi, he slept 'only' with the boss, old men who took him out of the street, and take him to his brothel (Naoru was an on street prostitute before), Naoru refused any other clients. I was kinda irritated because of this part, the fact that he had sex with that old man, even when he was already with Hiroshi, but at least it was totally physical. Of course once Hiroshi found out about it, told Naoru to fuck off
<spoiler> yes yes, as I mentioned, he left the prostitution business, it was right after last sex with that old boss, and Naoru was fighting to get Hiroshi's trust back, a lot of things happened later, about Hiroshi - he's totally in love with Naoru, but I'm waiting for more communication between them
<spoiler> I don't recall anything obvious, at least for now, there was only that Naoru's not blood related brother, but there most likely will be a different couple, Naoru's brother and that his old roommate ;) and yes, I also hate those out of nowhere 'love rivals' just to keep the series going :( and those pathetic 'love triangles' which have nothing to do with triangle at all, since usually there are 2 guys fighting over one guy, so it's more like a love corner
I feel like I've read this before, as I recall does the seme had a heart disease or something? Did he die?