Chapter 38, Itsumi took Kurohime up on her offer to flip her skirt because she was interested in seeing what erotic lingerie Kurohime was wearing. Seeking a sexual thing out is not the same as merely seeing it.
Making the promise in the first place is the issue. She knows Ruruka will approach it in a prevy manner.
Itsumi was happily fondling Mukai's chest before she snapped her to senses. She justified her actions with "If someone gets their tits you you have to at least touch them a little bit, right? It's just polite!" She gave no more resistance after Mukai started to grind against her leg. The last panel of the chapter shows that she was getting into it.
There were more photos than the one with the random girl (and that photo wasn't an issue). She hugs Mukai while slipping her arm under her sling and jacket and sits on her lap going much further than one would normally do for maid photos. And she let's Yanagi hold her in an intimate way. It's even commented on how daring Itsumi is being. Compared to the other examples this is a minor offense and is more of an example of how little boundaries Itsumi has.
Now ask yourself this, if Rin was there for all of this, would she really be okay with what Itsumi is doing? If Rin was in Itsumi's class, would Itsumi be behaving the same way?