It's my understanding they have sex in the book pretty much "off screen", i.e. it's not described in any detail.
For all the information you seem to have at your fingertips, I had assumed you had read the novels. There are two sex scenes that happen while Bella is still alive that do not happen 'off screen' and one once she has turned.
I imagine, though, they will be 'off screen' for the film to keep the rating.
The movie can have sex scenes and still keep the PG-13 rating. They just can't show too much.