It's particularly uncomfortable now that Steam has some... risqué game titles out there. I'm surprised there isn't at least more options on Steam to mark certain titles as private.
You can set your profile private and then play in offline mode, which should prevent people from viewing your profile and seeing what your playtimes are. I'm not sure if playing in offline mode stops Valve from logging / viewing this data, however. I'd be surprised it it does.
In terms of playtime I think offline mode does stop it being logged. My wifi was stuffed in my game box and about 20 hours of a game weren't logged by the time I got a new adapter and reconnected to Steam servers.