From Bull to Cuck: There Goes My Slut

Shion has a crush on Kazuki, her childhood friend. Unsure of how to make Kazuki interested in her, she has been asking Motoya, her coach, for advice. One day, Motoya finally pounces on an opportunity to get into her pants, and he believes it's only a matter of time before Shion is completely his. However, Kazuki has a secret neither of the other two know about… (Source: Irodori Comics)