Alright, I'll be more deliberate about it:

What are the costs of permitting any level of discrimination? Why shouldn't a service-provider, including a freelance/small-business one, be able to refuse service on whatever conceivable basis, such as race or gender?

If prostitutes and providers of similarly intimate services (e.g. masseurs/masseuses, actors, personal trainers) are to be permitted this, why should not others be?

On one hand you have paramedics. On the other hand, I've never heard of an EMS company that permits such discretion, and anyway paramedics have to respond to a lot of wasteful dud calls. Would societal cohesion be harmed? It is enough that no or nearly no employers allow it other than on the basis of improper attire or disruptive behavior. The self-employed can do what they like in this respect?

A stronger counterexample would be insurance companies, and the well-known problems associated with the discrimination they have practiced.

Might we broaden and abstract the discussion a little bit?