The idea is that the fake relationship conceals a real one. I was thinking of the real relationship being ongoing, but I suppose that it could be something that has ended. However, I do have problems with More Than Blue and My Husband In Law.
I've read the MDL descriptions as well as yours. With Husband-in-Law, I don't see how marriage would make the adulterous affair go away. In any event, a one-night stand seems too casual. With Blue, it might be okay if the true relationship was hidden, but at the very least the lovers would have to confess their feelings to each other.
(Every time I think I've posted a simple subject, it turns out to be complicated.)