That is certainly a good point. But, she wasnt really that phased when she found out. So i am not too sure how worried she would be about this. I mean she had sex in the next room. Surely if that sort of stuff getting in a book would bother her, that wouldnt had happen.
I think it coming out you had sex with your girlfriend would be a little different than it coming out that you were at an orgy, but you're right, she doesn't seem to care that much.
That said, she freaked out at the idea of Chika's relationship with Akira getting revealed. And at the time, she suggested that Akira break up with Futaba, rather than Chika. That had logic to it, but it's starting to fit a pattern of her taking Futaba's feelings for granted. Looking back, she also tried to push Futaba to tell Akira about her bulimia, and later pushed Akira to break his promise to not visit the gym. And now there's hiding Nodoka's book, and that she had sex with Akira...
Guys, is Rin actually a Bad Homie?