
I don't understand what Nagizuka did wrong. Can someone explain me that part?

Basically Nagizuka tricked Makoto into admitting that he was gay by asking if he has a boyfriend in a discreet way so Makoto would answer without thinking. It may not seem like that big of a deal since Nahizuka's bi, but coming out is a very important thing for people, especially in a place like Japan, and outing someone as gay is extremely rude.
Kotani's smile when Shishikura said 'I love you'