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.
I think that, if you went to Japan and said that American readers claim there's no conclusive evidence that the two MCs are lovers, you'd get some really funny looks.
The "Yes, yes, Americans are crazy. I mean, look at how they're handling Corona-chan! There's no doubt, they're bonkers!" kind of looks.
But that's just my humble opinion, of course.