I don’t think it’s uncommon for people to have a physical relationship with someone while liking someone else. I know a lot of people who have done that IRL.
So, I’m just going to explain why Dojin ignored Jihan although they both liked each other. Dojin knows that he is gay and has accepted that for himself. He comes from a very conservative family who have high expectations for him that he will disappoint if he lives his truth. He likes Jihan and knows that he likes him back. But he’s also aware that Jihan is a minor celebrity, still gaining popularity. In order to protect Jihan’s image he distanced himself as the consequences of a homosexual relationship for a famous person are far greater than his own ([potentially] losing his family). However, shortly before he enlists he realises that he doesn’t want to live without Jihan and they meet again.
What I think kills the story IMO is that Dojin‘s sacrifice ultimately is completely irrelevant. Like there are zero repercussions for them (Dojin and Jihan) being in a relationship. Which makes Dojin look like an asshole.

Is anyone else confused about why Do-Jin chose to do what he did?
Do-Jin was clearly still in a physical relationship with Hyeok even though he knew how he and Jihan felt about each other, which I don’t understand at all. I get he felt bad for Hyeok’s situation as he has no one else, but he still liked Jihan. I mean how do you get physically involved with someone you don’t have feelings for while you have feelings for someone else, and the someone else you know likes you back. Like wtf? Why? He clearly could’ve at least waited for Jihan or himself to feel comfortable with confessing. It just doesn’t make sense to me.