Responses
You could see it in older media, when gender expression wasn't really a thing unless you were clearly feminine/masculine. They were mostly thinking in binary with sexuality and gender, so everything in between was "fake". The spectrum is only recently represented and in LGBT community you can still hear that bi people doesn't exist, they are just whores or that they will definitely cheat on their partner. Biphobia is more common than you could think, and not only from cishet, but also within queer people.
Why are authors so against saying their character is Bi MC is obviously not 100% hetero so idk why bro felt the need to say “sorry to say but hes 100% hetero” like sure he has only dated women but he very obviously has feelings for ian/ is developing feelings for ian.. idk if the translation was off and they meant he only ever believed he was hetero but idk its just odd like if u like the same gender ur obv not 100% hetero