The caloric numbers (shown in the article below) are a bit lower than those reported out years ago. And if it includes the woman swallowing a guy's cum (at ~ 5 calories a teaspoon) that 21 calories burnt in 6 minutes of sex drops down to a net of 15.  

There are a lot of ways to increase the exercise benefit of intimate encounters during foreplay and afterplay should a couple be in a fun mood. A few that I have partaken in include unclothed ballroom dancing (a great way to enjoy a lady's curves and gracefulness in action), and 'mirroring' exercises while both are unclothed. Squats are a great exercise; try them while facing each other, or if coordinated enough, have her stand in front of the man, with him gently holding her hips, and do them. (Some yoga moves also are good for mirroring.)  If a guy is lucky enough to be with a light-hearted lady, you can find these two suggestions adding some frisky fun and opening up new avenues of connection that create even more possibilities for the next time you get together.  

Here is the newspaper article:

How much sex is considered exercise?

We all know sex has the added benefit of burning calories but can it really be considered a workout? In the past, a bout of sexual activity has been rumored to burn anywhere from 100 to 300 calories. Fortunately, exercise physiologist Jay Williams of upwave.com, got to the bottom of the rumor.

According to a study conducted by the New England Journal of Medicine, sex lasts six minutes on average and only burns about 21 calories. Furthermore, WebMD reports that 30 minutes of sex can burn around 85 to 100 calories, with a vigorous sex life probably clocking in on the higher end of that statistic. Sex also has many of the same effects on the body as an intense workout — your heart races, your metabolic rate increases, and you stretch and strengthen muscles.

But is there really such thing as the ultimate sex workout? “Sex burns a few calories, but frequent friskiness isn't going to replace the gym,” says Williams.

If sex doesn't really count as your primary form of exercise, why not exercise for better sex? According to a study in the Electronic Journal of Human Sexuality exercise frequency and physical fitness enhance attractiveness and increase energy levels. Research shows those who exercise are more likely to experience a positive perception of self, in turn perceiving they are more sexually desirable and may perform better sexually. Which exercise is best? A study published in the Journal of Sexual Medicine found that women who practiced yoga regularly experienced more signs of arousal, including more lubrication.

Want to last longer than the average six minutes? “Incorporating some endurance-boosting foods can also help when trying to kick it up a notch in the sex department,” writes Williams. To improve sexual stamina reach for figs, oysters, celery, blueberries, peanuts, bananas, garlic or even chocolate