The uke has many sexy partners
Hm, I'm not really sure how to explain this well. I wouldn't call it cheating due to the fact that they are not in a relationship, I guess you can call them sex partners. I'm pretty sure the mc has the feels for one of them (you will find out who.) The other guy likes the mc, I'm not sure if the mc likes them, but they have slept with each other. This is a horrible explanation, because I don't know if you want me to explain the story. So, I won't. I'm sorry, I tried, hope this helped. ╮(￣▽￣)╭
Haesoo is not in a relationship with either of them, so he is not cheating on anyone. The thing he has with his step brother is very complicated, they are not dating but Haesoo would not call them sex partners either. Both seme's know they are not exclusive to Haesoo. Also, the step brother has no clue as to who the other seme is.
This is a better explanation than mine, so go with this one. ╮(￣▽￣)╭
Does the uke have two sex partners or the uke is cheating on one of them? I haven't read this but pls tell me.