Honestly, I think there's two reasons. Some folks are uncomfortable with sex, and PWP probably doesn't allow them to avoid it. Still others might find it exciting and are either ashamed, or simply don't want to admit it. Then, of course, you have those folks that are intellectual snobs, and they'll argue that there is no high-brow content and, therefore, isn't worthy of being written or read.

Personally, I'm all for it!

Edit: so that means there are three reasons. :p