For a fortnight, 24 women and 22 men kept diaries of how often they engaged in various forms of sex.Link (Thanks, Olivia!)
Then they underwent a stress test involving public speaking and performing mental arithmetic out loud.
Volunteers who had had penetrative intercourse were found to be the least stressed, and their blood pressure returned to normal faster than those who had engaged in other forms of sexual activity such as masturbation.
Those who abstained from any form of sexual activity at all had the highest blood pressure response to stress.
Update: Ken sez, "Just because people who have sex are less stressed doesn't mean that the sex causes them be that way. At least as plausible is that they have more sex _because_ they are less stressed and recover from stress more quickly-- stress and staying steamed being a turn-off, in general, not to mention physiological effects of stress on performance, for men."
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.