DEAR MISS MANNERS: I was a guest at a dinner party where the host had an overnight guest staying. I needed to wash my hands after petting their dog, because the dog had visibly dirtied them, so I used the nearest washroom. It was off the living area where everyone was mingling, and I had seen other guests using it. While I was washing my hands, I heard the hostess rather loudly and passive-aggressively say to her overnight guest, “We need to train the others not to use your bathroom!” I immediately felt like such an imposition for using the (apparently) wrong bathroom. We weren’t given any sor…