i hate people that date another person when they havent move on ...

I dont think its that simple in this case....

True, depends on the people involved. Sometimes it acts as a support and helps them move on, others they just become miserable together. Here Yonwoo is not really doing this out of love and dedication. If he did, he would have stood by Taejoo's side when Yoon returned and helped him resolve his issues with Yoon.