Lol that's Korean drama for you. Instead of the mainstream, shorter dramas, if you watch the longer ones with over 100 episodes centered around revenge it gets so frustrating. So this manhwa seems to have tropes from those longer dramas. Tip, don't watch those longer dramas. I have wanted to throttle the tv out of frustration. It's like a very long angst like 90% angst then like 10% happy ending at the end. 1/10 of those dramas end up with a non-happy ending.
Also I believe at most he has only kissed the guy. He may have tried to do the do with the other guy, but he passed out drunk. Then when he woke up he thought that he did the do but if you really look at it the guy just went along with it and let him think that they did it since he wants to fuck with Sam's and Gyeom's relationship.
Waouh!! This is really the first time I read a yaoi ( or a shonen ai) where the main character is cheating and sleeping whith another Guy....have to say I'm really disapointed now, sure this manwa is realistic but I actually read manga especially BL manga to ,exactly, escape from all drama and boredom of reality. So I don't know for you guys, maybe you think that the storyline is still interesting, but i'm stopping here reading this (i'm not such a masochist ╮(￣▽￣)╭ )and I seriously don't think it will have a happy ending. So fuck you manwaka san (︶︿︶)＝凸