Sexfriend

Sexfriend focuses on the protagonist, Takabe Tomohiro who just happened to have his first sexual encounter in the nurse's office after school with his classmate, Hayase Mina. Tomohiro and Mina had been just friends until that fateful day. But was surprised at her attitude, he started to become addicted to the woman's body, and he continued the relationship with Hayase and eventually desires a real romantic relationship, not just friends with "benefits and cookies". With the school nurse's help, they were able to have sexual practice everyday.

From Wikipedia