As others have said, I think the way forward would be to allow the sex worker to refuse a client on an individual basis for any reason, but not for the BUSINESS to refuse a certain class of people for a racial, sexual, religious etc reason.

You could also refuse to perform certain actions that aren't part of your service offering. So for a straight female sex worker, who doesn't want to lick pussy, she doesn't have that particular action as one of the things she will do. As such she can tell the prospective female client up front "cunnilingus isn't one of the services I perform, if you want that, go elsewhere". It's not discrimination then.