How did this movie get a PG-13 rating with all the titties?

I was watching it going, jeez, between the sex scenes, graphic violence, and female nudity, they must have decided if they were going straight to DVD - and since the Book of Vile Darkness was also released as an adult's only supplement in 3rd edition - that they would release an R-rated film. I can't say how shocked I am that this got away with PG-13. I'm not shocked at the movie, just shocked that it didn't get an R rating.

I can't read books or even newspapers because they're written in prose, and that confuses me. :(