Wow. Apparently people are walking out of the theaters...
It appears to be mainly people who don't grasp the concept that an R-rated movie may contain violence and nudity. Probably people who think that a film about superheroes is always geared toward kids, no matter what it's rated. Probably the same people who think that animated films are always for kids, who aren't familiar with Spike and Mike or hentai.
How many people walked out of Grindhouse? Probably not many, because they knew what they were getting into.
I've walked out of one movie: Jay and Silent Bob Strike Back, because it was so appallingly awful that I just couldn't stay there anymore. I wasn't the only one.