Unfortunately, this series devolved into pure NTR logic a while ago with how Sakura's first reaction upon seeing Shinobu kiss Fumi was to straight up sleep with the guy that has had his eye on her.
Poor Shinobu even has a thought dialogue that says Sakura will definitely understand if they just TALK!!
FUCKING TALK TO YOUR PARTNERS PEOPLE!!
DAMN!!
Also, for Fumi, there's a case that she's doing this out of love for Shinobu and jealousy towards Sakura, but that just proves that Airi is writing this like a pure NTR work now.
Fuck logic. Thinking? What's that? Talking to your partner? Never done it.