If you are providing a produce or service that CANNOT BE OBTAINED ANYWHERE ELSE, then there is an argument to be made that the sellers cannot discriminate on the basis of certain factors. Otherwise, there is a principle that sits somewhere in the emanations and penumbras of the Ninth Amendment, and it is the called the "right of contract." It says that two (or more) parties can contract for any purpose or objective other than committing a crime, and the Sovereign has nothing to say about it. If I want to avoid serving Leftists, or Democrats, or public school teachers, there ain't a damn thing Government has to say about it.