Me, I think sangwoo's bi or could even be pansexual. I'm about sure Bum's a homosexual.
Bum at one point liked a girl sooo...yeah bisexual is the best possibility
It's supposed to be psychological. Truth is you can find the same sex attractive, and can have sex with them but at the end of the day want no relatio ship with them. It's also very possible that because of Bum's seductions and psychological outbursts and cries that he loves Sangwoo all the time have psychologically changed Sangwoo sexually.
Koogi said that Sangwoo is heterosexual, but he kissed and had sex with Bum a lot of times and we could see in the series that Sangwoo finds Bum attractive (maybe because he looks like a girl?). How can it be possible?!! To me Sangwoo and Bum are bisexuals!