I won't speak about my thoughts on prostituion today. However, in the past, it existed due to economic necessity. Indeed, it very well might be "the oldest profession." However, it wasn't simply an issue of there being demand. Looking at the whole of human history, females have only ceased to be second class citizens very recently (merci beaucoup Simone). Many women resorted to prostitution because either that was the best they could do for themselves, or that was the only way they could survive. Now, it is true that many women were taken advantage of. Used by men or other women, some did not have control of the monetary unit and, thus, their own lifestyle. However, there were women who used prostitution to vault themselves from lowly beginnings to positions of power. And when you live in a world dominated by men, power is what females are forced to worry about. Because no person could truly be free without some power over one's body, mind and some of her or his environment.
So, in the past, prostitution was truly one of the few things a woman could do to liberate herself from an unfair deal. Now, for a lighter view of prostitution in the ancient world, take a look at this.
This is part of the Kama Sutra translated by Sir Richard Francis Burton, one of my favorite historical figures. It details some of the writer's view on prostitutes of the day. :)