Onaji Zemi no Someya-san ga Sexy Joyuu Datta Hanashi.

The protagonist is in the middle of preparing for a presentation at a college seminar. Ryousuke Kamiya, a college student, was browsing porn videos while taking a break when he stumbles upon a video featuring Yuuka Someya, a girl attending the same seminar! When he asks her about the video at a drinking party after the presentation, he finds himself being offered sex in exchange for not telling anybody about it...?! (Source: DLsite, translated)