I wish that Sims 4 not only had markings for bathroom doors, but I wish you could designate a door as a bathroom door. I really hate, when you have a Sim over to your house, and he follows you around like a puppy dog, even following you into the bathroom. Maybe, this needs to happen, in order for them to learn, but in people world, people know a bathroom. Often, bathroom doors have locks on them. Some people even automatically lock a bathroom door, when using a bathroom. *Hinty-Wink*
How about, as a possible solution, having a Sim ask where the room is, when they first enter a house, then there could be a "Show Bathroom option" Maybe, until they are shown, a designated Bathroom door will not be passable. Maybe, the game could even determine this, by whether there is a commode in there. So, even if there are multiple doors, guest Sims won't use it as a shortcut, to talk to you in your bedroom, or kitchen, or however the house is configured.
Alternately, a Sim can just lock all available doors, when entering a bathroom, and only a spouse can breach a door that has been locked in this manner. Just a thought.