After mulling for a few days, I've determined some of the most problematic assumptions underlying 50 Shades of Grey. As I've discussed, it is about a young woman, Ana, who embarks on a submissive, bdsm relationship with the dominant and slightly older Christian.
E.L. James treats Christian's and Ana's predilections — domination/sadism for him, submission/masochism for her — differently. Christian, Ana and pretty much everyone else in the story perceive Christian's kink to be sick, disgusting, shameful and based in childhood trauma. James directly connects Christian's wretched past, including his rape at the age of 15 by an older woman to whom he was then submissive for 6 years [!], to his current deviant desires. Furthermore, Christian's sexual interests seem impossibly tangled with his self-admitted inability to love. Ana perceives him as so broken and abnormal that she wants to "fix" him by convincing him to love her. [Good luck!] Especially in the first book, sexual domination/sadism = pathology.
Interestingly enough, Ana's developing interests in submission/masochism are addressed differently. Ana responds with surprise and squeamish fascination as she tries out various kinks. However, she and all of the other characters do not regard her sexual interests as evidence of a sordid, damaged past. Ana may be puzzled by her excitement, but she doesn't resort to retrospective analysis to pinpoint what childhood abuse triggered her sexual deviance. [In fact, she doesn't seem to have much of a past at all. Booooooorrrrring.] She mostly accepts her submission/masochism without feeling the need to change herself. The first book makes it clear that there's nothing wrong with her.
I suspect that Ana's submission/masochism is naturalized and accepted largely because Ana is a heterosexual woman. She is culturally expected and socialized to be the passive, receptive partner. Submission and masochism seem like an extreme, but apparently still acceptable, extension of passive receptivity. Blarrrrrgh, essentialism.
I have no idea why Christian's dominance/sadism isn't naturalized, especially since he's a hetero man, culturally expected and socialized to be the aggressive, active partner. I'll let you know if I come up with anything.