Having full penetrative sex on film is an art form, and requires a series of skills known to a relative few, in proportion to the numbers of actors there are out there. Simply to maintain the correct camera angles,movements and dialogue is hard enough for most mainstream actors - the notion of pretending that you are not under hot lights witrh a camera inches away with upwards of twenty people watching you, and then surprising your co-star with some unscripted rumpy is pretty unlikely! As for professional pron stars, they are just that, professional - it's a whole new way of working, and most actors, indeed most people, are unable to provide the blend of control combined with exhibitinism required. the short answer is, probably no 'sex' scenes are real, which is not to say the actors don't enjoy them, it's just a Rubycon that most wouldn't cross in the course of a mainstream film scene.