Some people have casual relationships. I haven't read the book or anything but from the few spoilers I've seen, it's not specified if KwonLim is gay or bi. At 5 years olders, he could have already been having high school 'fun' by 17, not really focusing solely on his curiosity of Haekyoung who would have been 12 at that time. Maybe he met someone/people who looked like/reminded him of Haekyoung and had sex.
Ok. This makes sense.
Also, to think that while KwonLim likes him, he never thought it will be reciprocated because he knows that the very person likes somebody else. So, yeah maybe he went on exploring other things.
Ok the ML is, obviously, Kwon Lim, the eldest brother.
While MC is pinning for his friend, the big brother has been into him for a long time too. From what it seems, it started from curiosity to him developing feelings for the MC(or it could from the first time they met cos why would you take notice your brother's friend who is 5yrs younger than you if you're not interested with them). I'm not actually sure if KwonLim knows MC's feelings for his brother since the first time they met or while he was silently observing him.
According to some readers, this is more on about the development of the relationship between the two but of course there's the brother/friend that sometimes will take some screentime and would cause some "emotional rift" between the two. It seems most of the readers find ML to be lovable because of his character but also they find Haekyoung to be annoying because of his push or pull character. I think most characters here find ML difficult in terms of character but he's the type that's very different when it come with the MC.
Also, from what I read ML started acting because of MC. Not sure how true. But yep he's down bad for Haekyoung.
Something I don't understand is according to some reviews they say that while Kwon Lim has a crush on him for a long time, he is not a virgin. While Haekyoung, his first is Kwon Lim. But, they also said that for Kwon Rim, there's nothing else for him but Haegyoung. This is the part that I don't get.