beta
I just want to explain MY point of view.
I'm pointing it at the beggining so no one will accuse me that I believe I'm infallible (of course I can be mistaken because it's only my own interpretation).
Let's start with the fact that in stories there are some elements which aren't stated openly. All of us can remember (I hope) literature classes where we analysed, interpreted different texts, books, poems etc.
Some things aren't and often don't have to be said openly.
Feelings can't be proven like mathematical proof or idk blood test. It's not something material, physical. But how do we know that someone loves or hates someone else?
By their actions, by their reactions.
How do we know mother loves her child? She has to say: "I love you?". I don't think so. The actions are enough (for me at least). Her smiling gently, helping genuinely, asking about their day. Small gestures here and there.
As I said in previous comments I can be mistaken but I think Haesoo has romantic feelings for Joowon.
1. He has stayed with Joowon for years although it impacted his relationship with mother, one of the most important person in his life...
2. He cares and worries about Joowon deeply. He was completely shaken when Joowon wrote he hurt his hand. When the affair appeared he abruptly interrupted important discussion with his mother and went to clear Joowon's situation.
3. He was jealous not long time ago.
4. He desires him physically.
5. He thinks about Joowon, his wellbeing, their past while being with his new partner.
All these things added together CAN hint, symbolise he has romantic feelings for Joowon.
Spoilers
Add to all of this the newest raws (chapter 87) it's imo rather clear Haesoo still loves Joowon and didn't move on.
I guess he "broke up" with Joowon.... Because he just didn't see the future for them .... He was scared, afraid. He wanted to choose something realistic, accepted by society, his mum. In earlier chapters it also looks like Haesoo hoped Joowon would marry a woman, choose simpler path.
Their relationship has stayed the same for years. Their feelings unspoken. He wanted the break. He couldn't stand that situation anymore. But imo it doesn't mean that he doesn't love Joowon anymore.