At least with the separation and she's out of the house, she can then be w/the OP far easier and more often, which means that at some point, they just may get tired of each other. When they have to keep things a secret, it's the thrill of getting one over on others. If it is out in the open, eventually the euphoria of the "getting together" will wear off. They will eventually see each other's warts and imperfections in time.
