
From what I understand why they kept saying "straight" and not "bi", is because They never liked the same sex before, never got interested in them sexually either. So they never thought about being gay or bi. He become bi after liking this one man, specifically pnly this one man, but there's no guarantee he might like another man after that man as he always prefers women, meanwhile he probably have more than one girlfriend before and never have aroused/interested because of men. Rather that bisexual, I think those characters in bl comic might be closer to pansexual, where they liked person just because of who they are regardless of their sex. It's abit different from Bi, although very similar and might be mistaken as the same.
Or I think they're just romantizing a bit about "I'm straight but ONLY for you I'd do gay" which means, he will not be okay with another man thats is not that man.

Spoiler-ish: I bought the raws (my Japanese is only basic, so this isn’t 100%), but there’s a reason why he’s hesitant to pursue his friend. It’s briefly hinted in chapter 1 when he’s buying the tie and will go into it more.
It seems like he was traumatized by a “friend” who wanted to try going out (the ol’ “if it’s you, I think I could do it”). Suffice to say, it did not work out. So rather than not thinking bi people exist, it seems he’s more concerned with losing someone he cares about and getting hurt by someone confusing their feelings again. It’s more the MC trying to protect himself and it causing more pain to both unfortunately. Give him a chance, he’s a sweetie ╥﹏╥

maybe also because in asia, supposedly straight is the norm? and if kanda seemed straight, then he likely never mentioned anything to give off the idea that he likes guys and likely only ever talked about girls, so it's not wrong to assume hes straight. bi/gay people who hasnt come out will also intentionally give off the impression that theyre straight. so you really cant blame the top
this just pmo hooking up w other ppl when ur future man is infront of u …. do these ppl know bi ppl exist