Comment 2 for bug 1499675

I suspect your Mark account is in the "nopasswdlogin" group. This means login can be done without a password being asked.

You can fix this from the command line:
$ sudo deluser mark nopasswdlogin

If this is the case I'm not sure what has gone wrong but that user should have been removed from that group when the password was set. It may also be fixed if you change your password using the user accounts settings.