Onaji Semi no Someya-san ga Sexy Joyuu datta Hanashi.

Sinopsis

Ryousuke Kamiya, who notices while watching porn one day that his college classmate Ruuka Someya is starring in the video he is watching. When Ryousuke privately asks Ruuka about it, she tells him that she will have sex with him if he doesn't tell anyone. (Source: ANN, edited)