A New York Times Magazine article reports that while heart rates during sex have been studied, no one has thoroughly examined whether or not sex is actually exercise. This is crazy, because people love studies about how things they like are good for them! Sleep more, drink red wine, eat chocolate — all well-covered by the scientists.
So a curious professor of exercise science (note: this may be the profession I am least suited to) at the University of Quebec undertook the study to see how much energy was actually expended during sex. Anthony D. Karelis brought in 21 young heterosexual, committed couples and asked them to jog on treadmills for 30 minutes as a baseline comparison to the physical demands of sex. The participants wore armbands to measure calories and metabolic efforts while jogging, and then wore the bands during at least one sex act a week.
For an average of 25 minutes per session (including foreplay), sex burned 3 calories per minute for men and 3 for women. Karelis optimistically says that “sex can be considered, at times, a significant exercise.” But calorically, it's about halfway between jogging and chilling out.