It's not really "dumbing" her down. I'm with you that she jumped the gun, but it is in her character to make rash decisions she doesn't really think through, as she has down earlier by telling suspcious people about MC's disease or going into a love hotel with an older man.
It's in character, as irritating as it is, and it is one of her flaws, but it's understandable enough and she is prone to being a bit bullheaded and rash. I think what will happen is she will begin thinking what happened through and Yukari will do something to save her or risk himself and she'll forgive him.
If you look back the circumstances, I think she'll definitely have an epiphany that she fucked up at some point. In chapter 11, Yukari says he doesn't want to have sex with anyone but her and that's why he chose to masturbate to get out the stones back then. He also only cheated when he is either being assaulted or is hallucinating.
It is an ironic tragedy, though, as in his mind, he was being loyal. I think it's a little early for the breakup arc considering that they've only been dating for 6 chapters, but it's still sad nonetheless.
@DeanWinchester
No, they start dating in chapter 11-12. The events depicted here happened in chapter 15, so it's technically cheating, but he was hallucinating from dehydration, fatigue, and trying to survive so he thought that the witch was Ayana.
@arpegiuso @BZo1480