I mean, at least in this manga Shino being super against the harem makes sense. Of course she's going to knee jerk react when the entire premise is contradicting her own decision. She never could confess (and has directly said that she needs to do this to get over Naoya), and will eventually make a mistake that Saki and Nagisa will almost guaranteed see. Nagisa will probably take proof of it, Saki confronts Shino and forces her to confess, Naoya may or may not walk into the room at the same time. And then it'll probably be the three of them convincing Shino to join. Marika will then get super annoyed because she was there first. Nagisa is basically inverse Shino: slow at everything, bigger than Saki Saki, willing to confess and interrupt the original couple for a threesome. (And lets be fair, Saki Saki is totally the sort who'd let her join in . . . because Shino's boobs are smaller)