I just dont like the way how Stannis is tricked into this "I will give you son" like a drooling doggy. Though of it as a kind of same blood magic performed at the end of A Game of Thrones but aswell had thought of this "extraordinary" sexual bond between Mel and Stannis that it was purely without any emotions, hopes or lies. Tough and necessary deal for both of them.
(A Clash of Kings spoiler)
But perhaps were witnessing the "thing" which will make Stannis marching on King's Landing without Mel's advice and help in movie wise sense, we'll see.