I remember reading that a long time ago. That's why I can never understand the comments people will make about sex being such a good workout. It's so not. I think I burn more than 21 calories walking up the stairs.
edit- Actually I thought it was more like 100 calories. I don't think it's as low as 21. But STILL whatever it is, it's pretty damn low. Unless you get all 50 shades of grey, then I'm sure it's more. Just doing the white man's overbite? Not much.