As part of my day job, I've been consulting the endlessly entertaining Compendium of Physical Activities, which puts all human activity on a scale to measure physical effort and the number of calories burnt. The lowest things on the scale are, reasonably enough, sleep (0.9 mets), and listening to music (1 met). The highest is "Forestry, Ax Chopping, Fast" (17 mets). To save you looking it up, 'sex