Folks here passing judgment as if they never felt shame before. It's understandable to me. Mans was ostracized for something he liked and it resulted in becoming a (brief) social outcast. He put those feelings in a box and put that box away and tried to pretend to be someone else. It worked, sort of, but there were some cracks, and his true feelings bled out when he encountered Nanami, who was a social outcast. Next chapter will be him facing the same risk of being outcast again and making the wrong decision.
It was a poor choice, it was the wrong choice, but it's not like he did it to be spiteful. He was ashamed and afraid. There's no real information how he behaved following Nanami shutting herself away and upon her return to class which, if he continued mistreating her, I would be first in line with gasoline and a match.