(I will argue) the reason 50 Shades could be popular in the first place was because it is fundamentally anti-bdsm: it deploys it for titillation, but safely defines it as 'bad' and those who engage in such are 'broken' and everything is wrapped up and 'fixed' in the end (of the trilogy), so the audience gets to have the 'naughty thing' but get their previously conceived notion of 'it was bad and always will be so' re-enforced at the end.