Makoto Hirata is a pessimist who struggles to connect with people. His best relationship is with his childhood playmate, Mizuki. One day, by chance, Makoto witnesses the school "playboy" Hasegawa having sex with a girl classmate. Discovering a kind of thrill in the act of watching, Makoto continues to spy on him. Before long, Hasegawa has moved in on all the girls Makoto knew and cared about... and finally, with the only person he could call his friend.
Apparently the sooner you tell her the sooner she breaks up and dates the mc though she feels guilty about losing her virginity. However there are no scenes showing this so the most likely answer is this will show the too late ntr ending.
From what I remember, if the MC tells her immediately, he confesses his love for her and while she's upset with herself for falling for the other guy's lies and losing her virginity, the MC still wants to be with her, saying that if he would let that keep him from being with her, he never truly loved her, and she accepts. She showers him with love and affection every day and the MC hopes they'll be together forever, Happy ending.
The second ending is where the MC tells her what the other guy's doing later on down the road, after some of his "training" has been done. She doesn't believe him at first, but catches the guy with other women and realizes what's going on. The MC still confesses to her afterwards, but she's too shaken by what's happened and tells him that she doesn't want to date anyone for the time being. They slowly drift apart, and the MC realizes that she's lost her innocence and never smiles the same way again.
The third ending is where the shit really hits the fan; the MC begs NTR guy to let her go because he loves her, and NTR guy allows him to have sex with her while she's blindfolded. Evidently the MC's dick is so small compared to the one she's been getting that she doesn't even realize what it is, and she's so in love with big dick by that point that she rejects the MC. Later on, she and the other two girls the NTR guy has been boning (the class president and cheerleader) end up all getting piercings and doing livestreams together with NTR guy, i.e. the usual NTR ending you know and love.