We must be odd balls.
We very rarely shut the bathroom door and we have never locked a bathroom door in 40 years or so. If we are in mixed company, it's understood that a shut door means that the room is occupied a open door means that it isn't.
No, you sound like very logical, responsible reasonable people...perhaps a rare thing, lol! I can't imagine wanting or needing a lock on a bathroom door. If I lived or traveled with people who made me need a lock on the door you can be sure that would be my ONLY trip with them. Even little children can (and should) learn that a closed door means KEEP OUT. But, sadly many parents let their kids have free reign...just watch some time when you're at the mall, grocery store or WDW.