Yeaaahhhh... I felt that way too. I'm Bi and so many people I was with shoved this stereotype onto me. Made me feel a little sad to see it in the comic but I just wrote it off as him seeing the worst case scenario. So his bias and anger was coming out in it. I know I've been there and felt bad about it later. So I forgave in the end. :D
I think to truly understand Muyeong's character you need to understand why he has a fear of bisexual people. His first crush would literally violently use his body and then parade around with his gf an hour later. His fear and dislike for bi people (although biphobic) makes total sense considering his past. It's a reason but not justifiable
I'm at chapter 30 rn and some of the things in this story are so biphobic,, like 'typical bi: actually likes women more' maybe it's just the translation but idk