I think I'm slowly coming to agree with Blastaar's hypothesis.
For the record, I'm 100% with White Rose's hypothesis.
To wit: that these two have been lovers since the moment Kasumi agreed to share a bed with Sakurako (that is, chapter one) and that their relationship is fucking blatantly obvious to the Japanese fans.
The two theories aren't mutually exclusive--I think that the WR theory is essentially correct (for whatever value of 'lovers' anyone wants to assign). But Blax said it quite well:
especially with the author introducing more 'classic' couple who make all the doubter go "Why aren't the MC like that" when the characters are all like "I wish we were like them"
So my hypothesis is that the author's not trolling "us," the believers, but the "on-panel penetrative sex or it's not yuri" crowd.
I don't think the author is "trolling" anyone. She is just writing a cute atypical love story between two women that doesn't fit society's labels. I don't think the author could be any more on the nose about it than this last chapter or a few of the scenes we have had before where Kasumi and Sakurako actively discuss and ponder the nature of their relationship and specifically how it doesn't fit neatly under society's conventional labels such as best friends or girl friends etc.
I always find it funny to see the two primary camps on here arguing since I think both are completely missing the author's point. One side claims basically that these two aren't dating/lovers etc. because we never see anything physical between them beyond what could be passed off as platonic skin ship and thus they are just close friends and nothing more and the other camp which claims that they are basically fucking off screen every time the camera looks away and they are a typical lesbian couple and we just don't get to see the juicy parts.
Chapters like this last one discussing the nature of their relationship go to great lengths to hammer home the point that neither of these scenarios is correct and the deep bond and love Kasumi and Sakurako share simply doesn't fit classic labels and doesn't need to. Just because their relationship is depicted as sexless doesn't mean they are necessarily just friends and conversely it doesn't mean that they are just a typical lesbian couple who just happen to do everything sexual off screen.