I always pay cash where I can. It's often cheaper, and if they can get away without declaring it, good luck to them. The less tax gets collected, the weaker the state becomes.
I don't have any intention for the state to become weaker. It's just that I feel that it's too private and personal for the state to come along and assess how much it should get out of it. I don't think prostitutes should be taxed on what they do. Associated things like accommodation costs, running a sauna/massage type operation etc yes. But the actual sex, no.