
I apologize ahead of time if I took this the wrong way, but people who are bisexual aren't confused about their sexuality. They just have an attraction to both male and female.
BUT because they are high schoolers, that's be where they'd possibly be confused and questioning their sexuality. And I do agree that Mikuni was probably questioning if he did like Kogawa / a male partner, and it would've hurt Kogawa down the road if Mikuni realized he was straight. (But he blushed and almost kissed Kogawa so I think he liked him at least more than friends.)

In chapter 2 when Mikuni and Kogawa were talking on the roof at school Kogawa said "how would you feel if I said I liked you" and Mikuni replied "I would be happy". Then Kogawa said "What about kissing me?" Then Mikuni leaned in ready to kiss him. To break it down when Mikuni said he only like the girl as a friend not really romantically so he wasn't really happy when she confessed but the thought of Kogawa confessing made him feel happy.
In retrospect Mikuni liked Kogawa more then a friend at that moment but I'm sure with Kogawas rejection he took it as though he didn't feel the same.
the older mikuni is bisexual. and kogawa rejected him. i would’ve done the same if i liked someone who isn’t sure about their feelings for me let alone the same sex.
but did i miss something? how do people know the older mikuni liked kogawa?