As per the flight of the original poster - I think the FAs were extra jumpy probably because they knew about why the Air France flights were cancelled. Esp. since your flight was out of CDG. In light on the developments on this particular day, I can't say as I blame them - though their solution might have seemed tacky to you.

UA should just make permanent signs for the doors indicating what section the lavs are for (First Class pax; Business Class; etc.)