Wait a minute.... Did they really sleep together or it was an illusion that kihoon got(⊙...

They didn't I think its just an illusion.

I don't think they slept together. The blond guy is sored because he carried the Kihoon all the way home.
I think the fight was from the time they're a couple. Right after Seohyun rescued the blond guy (Hae something) from the bar, when he was drunk... Hae something was naked because Seohyun helped him I think he threw up after drinking a lot because of his ex. Kihoon was pissed because Hae kissed him when he was drunk and didn't want Seohyun to bring him home, even though Kihoon carried him.
All those scenes have gray background so I think it's kind a flashback.

I hope ╥﹏╥

Thanks(づ￣ ³￣)づ