Based on the erotic game by Orcsoft. 
 
See that quiet, plain looking girl who sits away from the rest of class? 
Sullen and glum, she gets talked about behind her back, but she's an earnest and tender girl. 
---Yes, only I know the goodness that's in her. 
---And yet, everything about her is a deception! 
 
"Sorry...... Basically, I just like...... to have sex?" 
 
Protagonist Kazuhiro Sayama tastes shock and despair watching his first love bang the despicable gym teacher. 
He dreamed of being her white knight boyfriend, and for that he's rewarded with the duty of documenting her f*ck like an animal with pretty much any guy. 
 
(Source: DLsite English)

Kanojo wa Dare to demo Sex Suru.