We have a one bathroom type situation. No stalls. Just one door and one commode (ick). Thank god we have a men's and a women's.
Question (based on real life experiences):
Who is it more awkward for when someone does not lock the ONE door? The person getting walked in on or the person walking in?
You then have to see that person periodically throughout the day. Avoid eye contact and definitely avoid conversation. You have that vision of their surprised face burned into your memory. "Oh....uh...sorry!" You both yell it. It's terrible.
Moral of the story. LOCK THE DOOR.