Part of the background I made for the fantasy world I built for the role playing campaign I ran a couple of years ago was a semi-planned attempt to examine where I thought a society might end up after several centuries with essentially universal access to highly effective birth control and medicine that makes sexually transmitted diseases rare and treatable and without the influence of any religious belief that sex is fundamentally evil. In the society I ended up imagining, whore is not a dirty word. It is understood that by providing physical comfort and a release of tension for people in need, whores are providing a valuable service to society. Further, whores in this society have a secondary function, to provide emotional comfort/support and counseling that complements the one-on-one counseling provided by priests. It is the whore's responsibility to be emotionally honest, to not pretend to be deeply in love with the client, but a good whore has to honestly enjoy the work and care about the client's well being.
Still, I realized, there is some ethical tension over whores. Some people feel that it is wrong to demean sex as a commodity, that to accept payment for sex reduces it to nothing but a commercial transaction. But I thought about it, and I imagined a person in this society who enjoyed sex, and further who liked people and enjoyed comforting people with sex, could spread their favors around freely as a hobby, but if they had to have a regular job to support themselves, they would not be able to spend as much time doing what they were really good at and really enjoyed as if they accepted payment. Both this individual and society as a whole benefit when this individual accepts payment. I realized that whether it was ethical to be a whore depended on whether the whore was doing something they didn't want to do out of necessity, or accepting payment for something they wanted to be doing anyway to have the economic freedom to continue doing it. Admittedly, it's a poor basis for judging others' behavior because it's pretty hard to know how someone else really feels about what they're doing, but the principle is there: it's ethical to accept money for doing something socially useful that you really want to be doing anyway. If you wouldn't do it if money weren't a factor, it's dodgy at best, and it's unethical to do something that carries so much emotional weight for others if you truly don't care about anything but the money or worse if you actively dislike doing it.
One night I was taking a walk, thinking thoughts along the lines of the last couple of paragraphs, and my thinking jumped to another subject: the ethics of breeding animals for money. It didn't take very long for me to realize that it was exactly the same ethical dilemma as the fantasy world whores: a person who really wants to raise animals must either be independently wealthy or accept money for the animals, but as long as they genuinely care about the animals first, I can't fault them for accepting the money. On the other hand, I find the person who sees breeding exotic cats or domestic dogs as a cold-blooded way to make money, for whom caring for their animals is just a matter of keeping that government inspector happy enough to not get shut down, as despicable.
The next jump my thinking took was the realization that authors, musicians, or any other creative artists were pretty much the same thing. If the only thing they cared about was getting the money, they were going to cut corners wherever they could. Creating is a fundamentally good thing, and there is something wrong about going through the motions to make money. (It is generally harder to get paid for a crappy book or song that you produced with the minimum effort and emotional investment than it is to get paid for a baby animal of a prized rare breed that you raised without any love. But I don't think that difference undermines my point.)
I quickly came to the realization that this dilemma generalizes to just about any acitivity that's worth doing at all, whether it's farming or doctoring or fixing cars or managing a big charity. If it's worth doing at all, it's worth doing well, with the kind of love and attention to detail that comes from doing what you want to be doing. If you don't care about the work, only about the money, it is just human nature that you'll only do the minimum that someone is making you do to get the money, and it's hard to get a good quality product that way and even harder to avoid hurting other things along the way. Only love and pride in the work you're doing will keep you always doing the best you can.
Thus, I come to the conclusion that the perfect world is one where no one is forced into prostitution. It's not a world where no one works, but it's a world where no one *has* to work, and no one is stuck having to work they don't like. A silly notion, perhaps -- but as our technology advances, material wealth is more and more available. Today, America at least is wealthy enough that no one should starve or live in the street because they can't find a job they want to do that they can get someone to pay them for. The hopeful technological future some SF writers imagine raises the world standard of living to the point where no one in the world should lack anything the nanofab factories can produce just because they won't prostitute themselves.
I guess that's the real conclusion. The evil of prostitution has nothing to do with sex. It's the evil of doing something that should be done for love for nothing but money.