Based on Boku wa Tomodachi ga Sukunai.

Three ugly otaku were sick of feeling second-rate. They loitered after school, grumbling about all the tail that good-looking guys got.
While ducking back into the class for some forgotten item, they overheard a sultry voice, and saw the girl with raven black hair masturbating alone. This was perfect. They chose her to expunge their pent-up sexual frustation.

[From DLsite english]