I had it the other day where there was just a single restroom for business class, after I politely asked them to go and use the Economy restrooms, the customer refused and kind of ignored me and went on in after I had explained politely I needed it for business class customers.
No sooner the restroom became occupied by the Y class person, someone from Business got up to use the facility and couldn't, so then had to sit back down and wait 5-10 mins for a washroom in their own cabin area, which would have been empty for them - as said person really took their time.
Isn't that wrong when someone has paid a premium fare? That's my prime reasoning.