I also had that happen, ONCE. I was SO taken aback, all I could do was stammer! And we are not friends anymore.
The way my house is laid out, the master bedroom is the farthest away room, with the master bath deeper in there.
The front bathroom, for the kids and guests, is at the front of the house, and I keep the lights on, if expected company, a lit candle, and fresh towels.
Once a year I host a large party, and I SHUT my bedroom door. That is where I stash stuff, also, and since the room is back and out, you really do not notice it.
There have been debates about when company sleeps over, that the MB is given to them. I grew up with the idea that the MB is the sanctuary, the marriage bed, the retreat, a safe haven, not to be invaded.
My kids can come in, but for a specific purpose, and they usually ask.
Even my best friend has only been in there once or twice, and we have keys to each others houses and jokingly claim each others' kids as our own.
Kid bedrooms are a hang-out place, for other kids, not adults.
My house has two living areas and a large open kitchen, so I guess I am lucky.
My master bedroom will be seen by others when they need to clean it out upon my death.
(Or if I ever get super organized and want to show it off...but I need to get the rest of the house ready, first! lol)