彼女が野球部の性処理マネージャーに… ～体育会系セックスで寝取られて～（Kanojo ga Yakyuubu no Seishori Manager ni... ~Taiikukaikei Sex de Netorarete~）

Since the protagonist Ryouta Tachibana enrolled in Kousei Academy,
he has concentrated on base ball practice for his purpose of the national competition.

His practice is very hard but supported nicely by a manager of the baseball club,
and his beloved one at the same time, Misaki Himekawa.

Because the baseball club prohibits team members from dating, those two has kept their relationship secret from others. But they are still able to nurture their love sneakily, making Ryouta's school life delightful and more than satisfying to him.

--- Until their relationship is found out by a new coach Gouda.

Knowing their relationship, Gouda forces Misaki into sex.
Misaki devotes herself to Gouda, because Ryouta's dream
to play baseball in the national competition is destroyed otherwise.

Gouda intesenly r*pes Misaki with an attempt to make her his own woman,
with his muscular body strengthened while he was a professional baseball player.

When a cunning junior boy Kobori comes to know about the relationship
between Gouda and Misaki, the ring of gangr*pe starts expanding to
encompass other members of the baseball club...

[From DLsite English]