To be fair, the only reason this info is public is the person is 'tagging' the activity. By default all tagged links in delicious are public and that makes sense. If you tag an activity like this on fitbit, you should mark it private.
I have a fitbit and I can't for the life of me figure out why people would start tagging every activity they do and start and end times... but if they are adding tags, comments, and other personal details they should mark it all private.
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.
I have a fitbit and I can't for the life of me figure out why people would start tagging every activity they do and start and end times... but if they are adding tags, comments, and other personal details they should mark it all private.