YANBU. I don't understand why anyone would want to be in the bathroom while I am cleaning my teeth. It's not like we can have a conversation is it?
Or would they come in just to blankly stare at me until I had finished? 
I really don't see the point of anyone else being there while I am cleaning my teeth.
If they want to use the bathroom, surely either they or I can wait until the bathroom is free?
What annoys me is when I mention I'm just going to clean my teeth, shortly followed by the sound of my brush buzzing along and at this point, the other person will decide it's now a really good time to ask me not very important questions, and repeat and repeat whilst I make Mmmm noises which translated is 'Wait a minute.' with my mouth closed.
More annoying than this is anyone, including DD who comes into bathroom whilst I am cleaning my teeth only to stand there silently staring at my every move.
Why????
After cleaning, I say 'Why do you talk to me/stare at me while I am cleaning my teeth? I can't talk back to reply to your questions can I?'
It still happens at least 3 days a week though. 