Why
people like drama lol
Some people think he treated him coldly. I think it was more realistic. You cannot have a rich guy fall for you in the second day he met you (or slept with you). If it was the case, he would have been married by now with his first sex-partner.
I think it was when he told him he was just a sex partner and not his boyfriend which drove a nerve with people. Most likely because many people have been in that situation, where you like someone and you sleep together for them to tell you that then don't see you as something serious. Even if you haven't experienced it, you can still emphasize.
Couple that with the fact that Yeomin has a baby face and he is so kind and optimistic, most readers feel a motherly instinct to protect him (since most of the audience is women and girls).
Who wants ML to suffer?
I love the way all of us readers are so united. We love yeonmin’s peace and want him to be happy, but we need ML to suffer at the same time.