I can't tell if this author hates women or thinks every woman is a dumb bimbo with no thoughts or will of their own. If Nagi had any thoughts going on in her brain this would be how her thought process should have gone:
Sakura: Shinobu cheated on me.
Nagi: The guy who wasn't even her boyfriend at the time and turned me down with his dick in my mouth, cheated on the girl he finally got together with? Something isn't right here. That's strange. He was really faithful to you back on the deserted island when you weren't even his boyfriend, despite the fact that I was flirting with him constantly.
Sakura: Really?
Nagi: Yeah. Give me his contact details. I want to hear it from his own mouth.
Sakura: Gives Shinobu's number to Nagi.
Nagi: Calls Shinobu. Hey Shinobu, it's Nagi, long time no talk! Yeah since the deserted island. Puts a finger to her lips to tell Sakura to shut up and turns the phone to speaker mode. Hey, so you finally got together with Sakura, but someone told me you ended up cheating on her? What the hell happened you scumbag?
Shinobu: I didn't cheat on her. She caught me in a compromising situation with a junior who had feelings for me and didn't realize I was Sakura's boyfriend at the time. Sakura saw that and took off running. I chased after her and tried to call her and explain what had happened, but she ignored me and didn't answer any of my calls or messages.
Nagi: Looks at Sakura increduously. Seriously? She didn't listen to a word you had to say, after everything you two have gone through and all the support he's given you?
Shinobu: Yeah, I was surprised she wouldn't even give me that opportunity. I later learned that Sakura turned to Yuma, who probably used her confused emotional state, to sleep with her. I really can't understand why she would immediately turn around and sleep with someone like that, and not realize that she was being taken advantage of.
Nagi: Looks at Sakura with a dumbfounded look on her face.
Sakura: Mouth is bobbing open and closed like a fish after realizing how stupid she's been.
Nagi: Damn that's rough. Do you still have feelings for Sakura?
Shinobu: I would be lying if I said I didn't, but the fact that she gave me no chance to explain, and immediately slept with Yuma makes me think there was already something going on with Yuma, and she used this as a chance to be with him, which really hurts. I don't think I can forgive her for that. Sorry I don't really want to talk about this anymore. Hope you're doing well.
Nagi: Yeah, I hope you are as well. Hangs up.
Nagi: Do you have any thoughts going on in that empty head of yours or do you just go with the flow no matter what?
Sakura: But I saw him kissing her!
Nagi: And instead of confronting him and asking what the hell was going on, you just ran away and then slept with Yuma? Is that really an appropriate reaction to the guy who has been giving his all for you since the start?