Not wrong to have feelings but it is completely wrong of him to continue to oush them in any way after being told straight up that it isn't mutual amd that they actually like someone else. Having feelings doesn't mean you get to try and charm the other person no matter what if they tell you no.
Yes that's why I said he should have done a better approach to pursuing his love. First and foremost is to respect the other person. He should have also gotten permission for his actions. Nothing wrong with his feelings though but feelings don't justify all his intrusive actions.
He's breaking the good relationship of the main ship but he's not wrong. He's assertive and brave for pursuing his love. There's also the part where Taejun actually let things become this way. Taejun should realize things and find the courage to get his man back. However, Hayoon should be pursuing him within boundaries. Perhaps if he talked and tried the initiative to ask for permission, this would be better. It's weird to have someone you rejected try to cling to you. It's the wrong approach but he's not wrong for having feelings.