Allegedly the writing in the books is really poor, and playwright Patrick Marber was brought in to try and salvage the script, but E James wasn't having any of it, so the film is stuck with the appalling dialogue. I think the whole premise of the story sounds really pernicious, but dressed up as a love story.
In my opinion you also need a good narrative to drive a book along, if you want sex and a brilliant plot I would recommend The Crimson Petal and the White