Theoretically. I mean, sex elevates the heart rate, and increases circulation - if that isn't aerobic, I don't know what is.
When I was at UCSD studying excercise physiology a couple of years ago, I remember reading that really vigorous sex burned about 600 calories per hour, or the equivalent of running 6 miles. The notion of how they arrived at that figure always made me laugh - did they hook up two people to calorie expenditure machines in a laboratory, and tell them to go at it?
"More thrusting, William, your heart rate is dropping!"