The best course of action for me is to presume that the person I'm in contact with has all possible imaginable STD. That's why I want everything covered.
Sex workers are in contact with many people, and a certain percentage will have an STD, whether they know about it or not, or whether they're being truthful about it or not.
I'm a bit puzzled at the reasoning that someone should be denied because they propose to engage in unsafe sex. The sex provider has a right to set limits, and the client can then choose to accept those limits or go somewhere else. If a client says that he's been cloistered in a monastery for the last 20 years, does that mean that he should automatically be believed? Of course not. That's why, if I was a sex provider, I would just assume that every client has every STD, and cover up in consequence.
Anything else is russian roulette. If 5% of the general population has some sort of STD infection (I'm being very conservative), then I would expect that one out of every 20 clients is infected as well. For an unprotected sex worker, then it's just a matter of time that she catches something and passes it along to most of her clients, despite all claiming to be STD free. Remember also that an STD takes time to incubate before showing lesions or symptoms, but can be contagious in the meantime.