The two couple are nice together (chapter 75 and 76 are ones of the best ever ("demo Yuu, Kiou no Yuu wa suggoku suteki datta-yo" is hardprinted in my head)), but I also miss when there was just Yuu and Mari. I hope we'll have both situations.
It's abvious from the begining that the power is in Mari's hands and not in Yuu's ones, but Yuu is supposed to give the illusion of the contrary. And since few chapters, she is not. I think a big punishment is comming from the master.