I think I would clarify that ALL rooms with closed doors are private. Dens, bathrooms, bedrooms. If the bathroom door is closed, either it's being used or out of bounds. Once that door is open(if someone finishes in there), then it's open again.
Now, if you're the kind of person who keeps the bathroom door closed (nothing wrong with that), then put a 'please knock first' sign on the door, or at least let your guests know.
Otherwise, if I'm in someone else's house, every closed door is a door that I Do Not Enter.