What does it matter? Profession, job, whatever.
Everyone has to live. People need money to live. Therefore (most) people choose to work. Some cannot work and therefore are supported by the state (as it should be in any civilised country), some choose not to work even if able which is not good or right, but then again they will never gain a better standard of living or pension funds etc.
What is really the point of arguing about semantics? Of course prostitution is work. Maybe not a profession by definition, but then again many jobs are not 'professions' - the people employed in these jobs still need to earn a living, just like everybody else.
There seems to be a lot of underlying resentment on here towards prostitutes, tax etc, blah blah blah. I'm with Smiths on this one - as long as you are receiving the services you paid for and the time you paid for (maybe give or take 5 mins at start and end), why should the WG's tax status concern you, whether or not she is a 'professional' or just 'someone providing a service / doing a job'? If you employed a seasonal worker , say a gardener/labourer, to do a job for you, as long as the job was completed to satisfaction and they didn't totally take the piss with charging for much longer than they were actually working, then you wouldn't care to dissect the in's-and-outs of whether or not he was a professional or just a casual worker.