If I'm in the flat by myself then anything can and will be done door open if I'm busy doing something and need to make a quick pit stop. If the girlfriend is in then I shut the door but don't lock it, she knows I'm in there and if she can hear the shower on then she also knows she's welcome to join me in there. If anyone else, family/friends/whoever are over then I lock the door automatically for anything including brushing my teeth.