I sometimes feel that Kasumi must be blood-related to Yasuda-sempai from this manga here.
Haha. It's amusing that you picked that scene as an example, because the exact same thing actually happens in this manga (in the pilot episode, the "chapter 0" of the series). Sakurako is thinking about how Kasumi is so beautiful and so popular with the guys, always receiving love letters and invitations and whatnot, and starts worrying: what would happen if one day some bad man tries to assault Kasumi? Would Kasumi resist and escape!? Or would her superhuman laziness stop her from even putting up a fight???
She decides to put it to the test, and mock-assaults Kasumi who is on the bed having a nap. Kasumi doesn't resist at all, and Sakurako breaks down in tears -- wailing that Kasumi is WAY TOO LAZY, and that one day a real molester will have her way with her, and it's all so sad and tragic, and and and... And, at that point, Kasumi cuts in and says: "What are you talking about? Why would I resist your sexual advances? If I was against it, I wouldn't sleep in the same bed with you!" Which leaves Sakurako totally flabbergasted, heh.
I've noticed a lot of people tend to forget that scene when discussing the nature of the relationship between the two girls. Then again, since it was in the pilot, maybe they don't take it as canon?
Rereading that pilot, I didn't find anything that go against canon. In fact, it should be the real chapter 1 of the series. It starts with Kasumi narrating the story, then the POV slowly shifts to Sakurako, and the conclusion seamlessly connects with the beginning of the first chapter in volume 1 (where the first page replicates exactly the first page of the pilot, only this time from Sakurako's narrating perspective) which then switches to a flashback.
In retrospect, it was kind of a waste of time, all the debate about whether Kasumi and Sakurako are or aren't lovers who have sex 'n stuff. I mean, thinking back, already in chapter 0, we had Kasumi saying that she was ok with banging Sakurako. "Or else, I wouldn't sleep in the same bed with you!" Those were her words, loud and clear.
I guess she could have added: "Duh."
(˵ ͡~ ͜ʖ ͡°˵)~☆