Sayuri is honest and upfront, which is why the whole "You wish it was you" bit didn't feel off coming from her. Honestly, I really liked the story of this and its prequel. I'm still pissed off about the mum, not because she's cheating, but because she's hurting childhood friend-chan with her stoic attitude.
Really rooting for daughter-chan. Somehow it struck me as odd that she doesn't seem all that concerned about the dad.
And hey, maybe the dad would be cool with having a "swinger"-like kind of relationship.