Ecchi na Mousou ga Kanacchaimashita.

Innocent-faced Yuki gets a personal trainer who sees in him untapped physical potential... Did we mention Yuki loves to fantasize, and his trainer is a massive tease!?

One day, Yuki visits a gym, having heard about the handsome personal trainer there. He's told himself not to think dirty thoughts, but all hope is gone when he meets the charismatic Masaki, whose allure sends him into a full-blown trip! First comes the teasing, then the sweet dirty talk, then the pleasure from his fingers, and finally, that great big...! Masaki asks Yuki if he's growing his hair out to hide those dirty thoughts. Wait. Is this really happening...? Before he knows it, Yuki's signed up to join the gym, and his life of nerve-wracking personal training sessions has begun!

(Source: Renta!)