Bitch ni Natteita Ore no Osananajimi ni Tsuite -Ore to Marin to Sex Friend | 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 has a childhood friend named Sakurai Marin, who has been with him for a long time and likes to play video games with him. He gets teased about his relationship with her, but to him, she is just an inseparable friend, and even if Marin, who has a lot of friends, already has a boyfriend, he would not feel anything.

One day, after playing games and chatting with Marin, he looks at the screen for a while and notices that she forgot to turn off the camera. He then sees an unfamiliar man in her room, and his childhood friend giving the man a boobjob. Later, he finds out that Marin knew he was watching, but does not care about it, furthermore, she tells him that the unfamiliar man, Shitaka Eiki, is not her boyfriend, but rather, a sex friend.

Not only is he surprised to see Marin having sex with someone else, but he is also shocked when he learns that she has several other sex friends. He tells her to stop doing such a thing, but thinking that he just wants to get laid, Marin gives him a blowjob, losing his virginity without being able to resist.

From then on, the protagonist becomes conscious of Marin, and does not want her to mess around with other men. He tries again to stop her from having sex with other people, but Marin thinks that he only wants her all to himself, so she offers him to become her sex friend. However, it is not the relationship that he wants, so he turns her down.

And so, the protagonist's anxious feeling for his childhood friend continues, while, as if to provoke him, Marin continues to show off, embracing her sex friends.