It seems like you're referring to a Japanese light novel or manga titled "Boku ni Sefure ga Dekita Riyuu" (The Reason I Got a Second Male Lead). Since I don't have more context, I'll create a short story based on the title.
Introduce friction, jealousy, and the constant threat of exposure to the central relationship. Critical Turning Points in Episode / Chapter 3
To view the complete, uncensored media safely without risking malware, check specialized Japanese digital comic/animation storefronts that cater to mature audiences (such as DLsite or FANZA) using the original Japanese title: 僕にハーレムセフレが出来た理由 .