I guess everyone has a right to accept or reject a sexual partner for any or no reason. And I don't see how that is any different if the sex is being paid for.
Having said that, I would hope that if a WG wanted to exclude any particular category of punter, whether race, age, or whatever, she would state it on her profile. And yes, like a previous poster, I have seen lots of black WGs saying no blacks, Polish saying no Poles etc, so it's not necessarily prejudice, rather "it's a small world".
I was once turned away because of age. The WG said she had an age limit of 45. This seriously pissed me off as there was nothing about that on her profile and I had a wasted journey and no plan B.
Of course it may just have been because she didn't like the look of me. I don't think it was that. Although I'm getting on a bit I think I scrub up quite well, though I say so myself, and she has since added the age limit to her profile. Also I have never had that problem before or since.