What if the Side Story is actually what comatose Jihyun is dreaming about after getting in a car accident. There's no way Chiwoo would ever forgive him and he knows it, so his brain is creating a whole different scenario, where the things he did were just a dream, used as a way of telling him he fucked up. So when he wakes up he knows he fucked up, but because he thinks it was a dream, it gives him a sense of hope that he can do better.
Not saying it's what actually happens in the story, just saying it would be a nice approach to it if you think about it. Also my fangirl mind is thinking about fanfics 24/7 ╮( ̄▽ ̄)╭
I already know... The bathroom is where you meet dangerous ppl... and apparently sex situations too hihihi (≧∀≦)