Responses
He's sleeping around because he 1 has tragic homophobia breakup so he's trying to be a bad guy who doesn't like relationships in order to avoid getting hurt. 2 he's playing around cause he thinks dongwoo doesn't like him and instead likes his sister so he's trying to forget him. 3 he thinks dongwoo might not be gay and is straight so he tries to forget this fruitless relationship.
Naww, this is the type of story that I always try to avoid. I really don't like the other person still sleeps around even if he already made up his mind to make his love come true.