Trust me guys, the way Taehwan reacted is the same way I react if someone gets close to my younger sibling. You see the thing is, he is not homophobic, he is just an older sibling. I can say from experience, my sister(younger) and I had a common friend(a dude). I noticed him getting a bit too close to my sister, and I can't explain the feeling. It aggravated me so much. You see, from the start older siblings are expected to take care and protect their youngers. When you see someone getting too close to them, you get conflicted. It's hard at first but with time, you grow up. It's human nature to get jealous. Even I couldn't gut the fact that some boys were getting close to my sis, in ur mind you always have the fear that strangers can harm your younger ones. But with time, I realized that my sister has grown up, she can make decisions on her own and that my opinions shouldn't influence her. It's just normal human psychology. It's the same as dads getting mad when their daughters get bfs or moms doing the same.
At the end, Taehwan's reaction is a bit extreme but it is justified to a certain extent.
It's normal psychology, yes. But in most cases you would ask the defendant to leave the room/place for a while, so the offender could decide what to do further. On the other hand, Taehwan threw their friendship out of the window in 3 seconds. "Get out of my place, break up with my brother, never talk to me again". Quite an overkill isn't it? I'd say there's not only overprotectiveness involved but it's heavily laced with homophobia.
I don't really think he's homophobic, it's just that a friend you trust making a move on ur lil sibling is just kinda wrong. Even I would feel triggered if by best friend made a move on my sis. I think older siblings will kinda get it cuz you know, overprotectiveness ╮( ̄▽ ̄)╭
Edit: I will definitely agree that he's being too rude about this
I think she is that girl who liked Yu Yang