Personally, I don't mind the Nice Time for Roo. Why? Because the way the story is headed its obvious the MC is going to cheat on her countless of times... if she does it too then she will be just as guilty. Besides, if the demons have it their way, the world is soon going to become a place a pure hedonism, so I assume "free-love" is going to become a common place thing.
That only really works if the player actually makes Simon behave that way. You can have Simon be completely loyal and the game shouldn't punish you for this. I think the way to handle it is to have the two talk about their relationship in the context of Simon's servitude, with the option of making it a more open relationship out of necessity, to be loyal solely to one another, or have the relationship steadily deteriorate due to Simon being unfaithful but not allowing Alexia to do the same.