TheBoost said:

What boggles my mind is how something so dull and awful become so popular. Is our bar for 'lurid titillation' so low?

According to an article about the movie (trailer)

So who is the audience for this rom-dumb? Mostly women over 50, and housewives home with the kids. And that 20% of men who must be even less sexually satisfied than the bored housewives and women over 50.

“Fifty Shades” author E. L. James knows her audience since she herself is a 51-year-old lady from England who looks like she would enjoy a nice beating from “Poor, f-ed up, kinky, philanthropic Christian.” Or any sex actually.