I do remember Nodoka assuming that Yumi likes sensei and if you've done her route right, Yumi would not react much and just go on with what Nodoka's demands after she blackmailed her. It's either she doesn't want to betray Chika or she's uncomfortable seeing sensei commit adultery which we can assume is that because she cares about Sensei, but not specifically in a romantic way or it can be both. She just can't show it.
Edit: And while Sensei did gave her detention, it's not really abuse. It's just a disciplinary response by a teacher. Other than that, Sensei's been pretty nice to her all through out other than Akira lusting for her secretly, but that's not really the point cause Sensei mostly thinks with his dick sometimes. While some people remember only the bad things about their abusers, it's not the same with Yumi. She realized that sensei did more good things than bad to her all this time and that's why she was disappointed that Sensei didn't do anything when Nodoka blackmailed her and with it being confirmed that she's basically just a teenager who also wants to be desired, I think she look up to Sensei especially because he's the only adult who willingly supported her and tried to understand her situation.
For the longest time, the way she reacted to that kiss and her repeatedly bringing up the detentions led me to believe that pre-loop Sensei had done someething abusive in the detentions. But when she finally opened up about them, it was him constantly telling her she had potential and wasn't living up to it.
She hated him because he tried to inspire her to be better.
Yumi has shit parents who neglected her, so her self-worth is essentially set to zero. And she doesn't trust men, either, because the man who should have been a father figure is a shit person.
So when Sensei shows up and tries to inspire her to be better, she assumes the worst -- that he's just lying to do to her what her father did to Yuki, and she perceives everything about Sensei through that lens. And let's be honest, she's not entirely wrong.
All it's going to take for her to fully remove that armor is for her to come to accept that her fate is inevitable, that she's her mother's child and will never be anything more than that. When that happens, there will be no reason for her not to take the moments of joy that are made available to her.
The only other way she winds up hooking up with Sensei is if she comes to believe that he's the person Chika sees him as, which would be a very long process and his trying to juggle 35 simultaneous girlfriends is going to make it too easy for that to actually happen. Chika's naive and sees the best in people. Yumi is jaded and sees the worst. If this is the path Sel's set her on, she'll be one of the last girls to give in.
