Have you ever seen a grown man naked?
Are all "Rated R" movies created equal?
To me there can be a world of difference at times and some PG-13 movies are more troublesome than some Rated R.
Talking about television and movies, John Piper share's a great insight.
"I have a high tolerance for violence, high tolerance for bad language, and zero tolerance for nudity. There is a reason for these differences. The violence is make-believe. They don’t really mean those bad words. But that lady is really naked, and I am really watching."
(full article; cf. Warning: This Bible passage is Rated R by Robert Hall)