I'd like to know which PG-13 movies you've been watching. The positions are very graphic, and while I, personally, am not bothered, I can see where the problems arise. Though I'd have a far FAR bigger problem with the violence, if I were a parent.

edit: Those sorts of scenes in a PG-13 film would most likely be in the dark, and would not show any sort of positions like that. There are sex scenes in PG-13 films, but positions like that would earn a film an R rating.