Unsophisticated and Rude(2005)

Junjou to Furachi

Hiroto, a student at an all-boys' school, listens as his childhood friend, Satoshi, confesses that he is falling in love with Nao, an upperclassman in their school club. But Hiroto is perplexed that the object of Satoshi's affections is Nao - the same Nao with whom Hiroto spent an evening!

(Source: June)