Saenai Boku ga Kimi no Heiya de Shiteiru Koto wo Classmate dare mo Shiranai

Yuki Toyama, an introverted loner, has a secret he hasn't told anyone. It is that he and his classmate Yuzumi Takai are "sex friends." It was supposed to be a purely physical relationship. But when Marika Uehara, a popular girl in the class, began to show interest in Toyama, their attitudes began to waver.

(Source: Sneaker Bunko, translated)

Note: Chapter count includes 3 epilogue chapters.