The Twilight series itself is silly juvenile pulp romance and 50 Shades is the even more ludicrous fanfiction based on that.
Having said that, I think 50 Shades is for many woman what the Transformers movies are for many men: its silly and ridiculous, but then again there's robots and explosions. Or maybe Pacific Rim is a better comparison. Its childhood goofiness made (semi-)real. Everyone knows they're not seeing great cinema, but robots and monsters? Sure, why not. Twilight and 50 Shades are many women's juvenile fantasies visualized. And I think most women know that, but then again, where else are you going to see that on the big screen.
Also, we all know World of Warcraft is not the pinnacle of game design, and yet we've all played it. And we've all played it for much the same reason: our friends were playing it. 50 Shades is a safe outlet because its commonly accepted that everyone "had to read it" or at least "check it out for a few pages." You're not a pervert for reading 50 Shades, you're just one of millions of other women. We all have guilty pleasures (Silk Stalkings, anyone? Anyone?) and I'm not one to judge. Still not going to see it, because eww, but not going to judge.
On the plus side, I hear it gets that Frozen song out of your head for a couple hours.