"Madonna, who wrote the book on sex, thinks 'Fifty Shades of Grey' is 'Not very sexy,'" according to HuffPost. Really, that's what they said, and apparently they think that's such a clincher, it's worth being the title of the article.
One, while Madonna may have written a book on sex, she didn't write, say, I don't know... The Kama Sutra, which, if there's a singular book about sex, should be it.
Two, this is following the assumption that everybody has the same tastes on what is good, what is bad, and what is sexy.
Three. another assumption: that celebrities know best. Just because she may have lived a particularly raucous lifestyle doesn't mean she's a confirmed spokesperson for the topic. She's welcome to express her opinion, but by no reason should you allow that to (a) decide whether or not you should see the movie, and (b) how you feel about it.