Here’s the quote from Bill Condon and Melissa Rosenberg talking about Robert & Kristen in an interview.
Filming a Real-Life Couple (Pattinson and Stewart)
Condon: It was entirely a relief. I can’t imagine doing those scenes with two people who don’t like each other. But did I have to adjust the way they make love to each other? No, it was really good.
Rosenberg: I created the sex scene and then Bill let the actors go, and I don’t think Rob and Kris needed any help with how to perform. [Laughs.]
Source: The Daily Beast