that option has been disabled.
i'll tell you why:
1. we don't need girl friends, friends, and non members accessing the site when you step away.
2. for better security and what not, it's just safer this way.
3. more security will entail you to have to put up with logging in and out.
be creative, try not loging out and stay logged in if you are going to come back to your system in a little bit of time.
if you stay inactive, your session should time out (log you out) too.