Well, he's also cheating on Chika 24/7, he cheated on Futaba, and he blatantly wants to get in Yumi's and Touka's pants. All of which are his students, that trust him, and are minors.
He also only helped Sana so that eventually he could screw her, albeit he's seemed to see her as forbidden fruit lately.
Now, if she knew he's done stuff like almost rape Molly, or pushed Makoto to suicide, on top of letting Nodoka bully her and forcing himself onto her, or optional stuff like letting Kirin traumatize Ayane, joking about having a threesome with Chinami, and screwed his own niece, then I don't think she'd see much good in him at all. He also more or less confirmed how shitty he is on the recent beach event, to her (if she remembers that).
Yumi ironically sees him as he is, a predator. I can see her just accepting that part of him eventually, like Maya, though. (He might have even been planning on cheating on Maya with Yumi before the loops started, because of that.)