The idea that his lordship wants to be in the bathroom while you're on the loo or taking a shower, just to brush his teeth, makes my flesh crawl.
It's creepy, intrusive and controlling.
If he is genuinely in need of the loo and can't hold on for 15 minutes, all he has to do is knock and call out ''sorry, babe, I need a wee and really can't hold on any longer''. That would be quite reasonable. His behaviour is ''not normal''.
Tell him, ''OK, you can come in while I'm in here, just to brush your teeth, but you'll have to put up with my sanitary pads and the period blood all over them that I've laid on the loo seat ready''.
Also, tell him that if he really does need the loo, then he should buy himself a commode and keep it in the shed or whatever and take his teeth cleaning stuff in there too!
Please, keep the door locked. You have every right to privacy in bathrooms, the same as everyone else.