Toshoshitsu no Kanojo: Seiso na Kimi ga Ochiru made THE ANIMATION

Yukiha Serizawa is a newly minted high school girl, who enjoys working as a library assistant with Aida, a boy she likes. But things are about to change for her...

One day the school janitor asks to see Yukiha, to thank her for helping him the previous day. She trustingly goes along with him. The next thing the pure Yukiha notices is that she's been given a drug that made her pass out, and the janitor had his way with her. She wakes up in the middle, and finds she loves the rough sex she's experiencing. Although she wanted to save her virginity for Aida, she finds she can't resist the allure of sex.

(Source: J-List)