A need to go to the loo often counts as a disability. Businesses and service providers must, by law, make 'reasonable adjustments', wherever possible, to enable those with disabilities to have the same access to their services as people without disabilities do.
As I see it, simply ensuring that a loo is sanitised after a client has used it, would count as a 'reasonable adjustment' for a dental surgery to make.
Our local doctors' surgery has kept both of their loos available for use during Covid, with many people visiting the adjacent pharmacy (who aren't even using the surgery itself) often nipping in there for a pee. To the best of my knowledge, all supermarkets with loos have kept them open as well. I can't see where the difference is meant to be.