Regardless, they did it for reward, and going by that argument, whatever they did to get what they exchanged (traded) for prostitution is an older proffession than prostitution.
Okay, I see where you're going with this now. However, your original argument that hunting/gathering was a "profession" is still wrong. Reward and profession are two different concepts as you cannot call hunting a deer to feed yourself a profession. And trading your deer meat one time for six with a prosttute, also does not make it a profession. However hunting deer all day and selling/trading the meat
could be considered a profession. You see the distinction?
However, as Neko_Kineko said, "Prostitution is the world's oldest profession" is just a saying. I doubt anyone knows what the first real profession was.
Nice and realistic idea, but honestly (and I mean no offense to OP for this) it is a waste of space and would just cause unecessary uproar amongst people who think their 13 year old children don't know what six is.
And by that logic taverns are a waste of space because some parents think their 13 year olds don't know what getting drunk is? People are going to complain about anything, so why call something a "waste of space" just because someone might complain?