To quote an official MPAA movie ratings poster: "THE SYSTEM CAN'T WORK UNLESS YOU UNDERSTAND THE SYSTEM."
How many NC-17-rated films did you see in the theater in the past year? Maybe one? That is if Ang Lee's Lust, Caution even played in your 'hood. And considering the most screens it ever played on in any single week was 143, I highly doubt it (understandably, it could have played in more than 143 locations over the course of its 20 weeks in theaters, but I doubt many more).
But if there had been more NC-17-rated films, and they actually played near you, would you have gone to see them? And if so, why? Because you expect something more titillating than the other releases to choose from? And if not, why? Are you afraid of others thinking you are going to see something dirty? Are you embarrassed about both attending and watching graphic sex on screen? Do you correlate the experience to going to a porno theater?