Renai Kyoutei: Nukegake Nashi!

Childish student Shirou carries a torch for his teacher, Oka. Meanwhile, his good friend Masami—who has cherished him like a brother—confronts their mutual friend Ryuuji about his true feelings for Shirou. In reality, Ryuuji loves Masami, but pretends to love Shirou as well in order to attract the attention of the one he loves. Masami and Ryuuji come to a gentlemen's agreement, in which neither of them can hit on Shirou. Ryuuji orders Masami to substitute and take care of his sexual desires in Shirou's place. (Source: MU, edited)