When people say, “I have nothing to hide.” I remind them that they have a door on their bathroom and curtains over their windows for perfectly innocent yet no less valid reasons.
This was my 'go to' question for several years but it wasn't long until I met someone (we're actually now good friends) who gladly leaves the loo door open and do whatever in full view of anyone. From there I ended up with sturdier concepts, some listed below.
I don’t do anything illegal in my bathroom, but damned if I want a camera in there.