
ML = Male Lead
Ml fell in love w FL when he still thought she was a guy. He didn't care, he just fell in love with her. Aka he really could be bi. tbh, more like pan since he didn't care about gender, he just fell for her, even though he thought she was a guy.
Also, Bi= liking guys and girls.
Ya, Kexin fell in love with FL when she's dressed up as a guy, but that's only the "liking the other gender" bit. She, so far, has not found out about FL real gender. For all we know, if she did find out, she could have still been in love with her.
There is no proof saying she doesn't like the other gender.
Heteronormativity may make you want to think that they're all straight, but like, they probably aren't.

My bad, I confused ML and MC; I know what bi means, I just doubt greatly that they're bi.
I'm pretty sure they're straight since in this kind of manhwa, all characters are straight, although I would give you that ML seems pretty bi since he was excited to "sleep" with MC before he knew she was a girl but Kexin is probably straight, she fell in love with Mc as a dude, there's no hint or referring showing she might also like girls but we can also say otherwise, like you said, there's also no proof she doesn't like girls.
I just think that we shouldn't just rush in our assumptions ╮( ̄▽ ̄)╭
Kexin and ML are both Bi.
Convince me otherwise