I pity misumi ╥﹏╥
I think he does know why. I mean he seemed to have listened to the whole conversation and that is proven by the fact that when the first crush appeared in the restaurant, Sei-san knew who he was. So I don't think Sei-san is acting that way because he doesn't know why, it's more like he is acting that way because he thinks that Misumi is hurt and won't open up to him so Sei-san is being an idiot and protecting his own heart from breaking by breaking Misumi's
you have a good point but the way his acting gives the impression of not listening the hole story.
i'm super happy this got an update. but i'm sad Sei-san is been an idiot. Dude i know Misumi said that i doesn't what to fall in love with a straight man but fuck at least ask why and don;t shut him out.