When I saw the headline, I wondered how the machine determined that you were having sex, rather than just doing (for example) pressups, squat thrusts and crunches (possibly in close proximity to another user doing similar exercise). It would have to be an exceptionally subtle piece of accelerometer/resistance meter interpretation to distinguish the difference.

wastedbrains' reply told me exactly what I needed to know. Fitbit users are not "unwittingly sharing details of their sex lives", but deliberately marking their exercise sessions with their very personal trainer as games of hide-the-sausage; using a tool that is designed to publicly share information about ones exercise sessions.