Bijo to Yajuu: Gal to KimoOta 
Nanase is a beautiful gal who feels unsatisfied with the sex she has with her outgoing boyfriend. Out of irritation, she takes out her frustration on Nozaki, a creepy otaku on the same committee who keeps staring at her legs, and forces him to show her his dick! Nanase had assumed it would be small, but Nozaki's was huge, incomparable to her boyfriend's!! Giving in to her sexual desires that were building up day by day, Nanase tries inserting Nozaki's dick, calling it a dildo!? As her pussy gets filled up, she forgets about her boyfriend and can only think about pleasure――!? 
 
(Source: PinkPineapple, translated)

Seiyoku Tsuyo Tsuyo THE ANIMATION