Prostitution is a business, but the nature of the industry means wgs are often putting themselves in a very vulnerable position, alone with men fuelled by testosterone and I personally feel it is acceptable for them to choose their customers.
If they refuse to see black guys, simply because they are black, yes this is racist.
If they refuse to see old guys more than twice their age, yes this is ageist.
If they refuse to see women, yes this is sexist.
My personal opinion: In all the above cases, I think they have the right to choose, and it saves everyone's time if they are clear from the outset.