
It's not fully explained yet. They first met in high school in America after Yeonwu transferred there as part of the exchange student program.
(Speculations) They probably kept meeting up and I assume that Yeonwu confessed to Teahwan and got rejected. Maybe he called him something along the lines of 'diagusting' since Teahwan is a real homophobic as stated in the author's note.
Teahwan kept ignoring Yeonwu but something happened and made Teahwan kind of a bully.
(current timeline) They now have a toxic relationship, Yeonwu being struggling with an unrequited love but gets easily excited with anything has to do with Teahwan (even a mere stare)...

The BDSM is from the current timeline.
It happened when Teahwan knew about his brother's relationship with Dojin and went out a whole night.
I assume he spent that night at Yeonwu's place and probably vented out his anger by bullying Yeonwu (aka doing BDSM stuff). This is where Season 1 ended.
The author confused me too with that imagination part. What I gather is that the kiss didn't happen... the sex could be all in his head too bc his meeting up with that dude happened in the past.
can someone explain to me when the thing between them happened? i kinda don’t understand the timeline...