For a 160 pound male burns 200 calories on average per mile of running (10 minute mile). Sex take signifigantly less energy than propelling a 160 pound body across the ground at an average of 10mph.
With that being said even vigorous sex would use a fraction of the energy running takes to run a 10 minute mile. According to a recent poll, the average sexual encounter lasts about 10 minutes. So I would guess somewhere around 50 calories or less for a man and even less for a woman (since most female sexual positions are much less active then male positions).
So on average, no enough to consider a real aerobic exercise. But again, these types of questions are so relative.