ビッチになっていた俺の幼馴染について ～俺と真凛とセックスフレンド～

The protagonist is shocked not only to learn that Marin is having sex with someone else but also that she has several other fuck buddies. He urges her to quit fooling around, but Marin calls his bluff, suggesting he's just itching to get laid himself. In a bold move, she gives him a blowjob and takes his virginity while he's unable to resist.

From that moment on, the protagonist can't shake Marin from his thoughts, and he grapples with conflicting emotions about her promiscuous lifestyle. He makes another attempt to persuade her to ditch her numerous sexual partners, but she accuses him of wanting to monopolize her body. She proposes he joins her roster of sex friends if he's that keen to jump her bones, but he adamantly declines, insisting that's not what their bond is meant to be.

Afterwards, the protagonist continues agonizing over his relationship with Marin while she begins to flaunt her intimate encounters with her sex friends at every turn as if deliberately provoking him.

[Translated and edited from Getchu]