I don't view the trafficking of human organs as a good example of why prostitution is wrong AND should be illegal. You seem to be selling this general 'Selling your body is bad, mmkay?'' angle. "This seems similar to this other thing, so both are wrong!" First of all, why do we consider selling our organs to be wrong? Well, obviously there are certain organs that we can't live without, and if we can find those organs on the black market, then it probably means that somebody died to 'donate' those organs. There's also the fact that people have to endure a waiting list to receive live-saving organs, and circumventing this process can be considered to be unfair to those individuals who are abiding by the law.
Prostitution doesn't raise the same moral questions for me that black-market organ profiteering does. 'But they're both examples of body-selling!' doesn't work for me, because you still need to prove to me that 'body-selling' is wrong, and it what context. I'm not going to abide by some absolutist 'just because' line. The similarity between the two is only skin-deep, and only relavant to anyone who stops at their primal emotions.
I do not dispute you taking exception to this. While I would call prostitution analogous to selling one's organs in kind (if not degree) to selling one's own body, I actually consider prostitution
worse than selling one's own organs. The reason I consider it worse as a moral issue is the source of the
demand. When it comes to the organ trade, you are right: the people to whom the organs eventually go to have generally done nothing fundamentally wrong (usually) by being placed in a position where they need an organ transplant to survive. And unless they take a direct part in the organ harvesting (such as directly paying the harvester or kidnapping an unwilling victim), they bear little moral culpability for the ugly trade beyond merely being the recipient of the transplant.
Prostitution on the other hand is an occurrence that is created by a far more sinister demand. Unlike organs, men do not
need sexual release from women immediately available to them; they simply want it. I argue that prostitution is driven, at heart, by a fundamental belief held by many men that women's bodies* are fundamentally commodities which they should be allowed to buy, sell and lease for their pleasure with the only limit being how much they can afford at any given time. The fact that this insatiable demand leads to the victimization of countless young women is a fundamental evil fueled by the desire of fundamentally evil men. And when I say "fundamentally evil," I argue that if one seeks out pleasure with the full knowledge that someone is being harmed in the supplying of that pleasure, that person is evil. If you or I were to seek out a prostitute, and we did not know whether that young woman was a "free" and "empowered" young lady, or was the victim of trafficking, or we knew she was trafficked but purchased her services anyway, we would be utterly evil men for doing so. What is more, even if she wasn't trafficked, by purchasing her services, we would be contributing overall to a system in which women are trafficked like commodities in order to undercut competition and lower costs in order to allow men to buy the prettiest girls at the lowest prices. The only possible way to be moral is to not put one's money into the system at all, and to seek a willing partner who will freely offer you her affection after you have earned it.
Further, by decriminalizing prostitution, we are implicitly (if not explicitly) telling men and our sons and our future sons that it is perfectly alright to objectify women and their bodies as commodities of pleasure which they can freely lease or purchase. If one's personal goal is to create a community in which men respect women and women do not have cause to fear men, I cannot think of a worse avenue to go down, short of re-enshrining the right of men to own women as slaves.
*Note: I should include young men, but it is mainly young girls and women who are the victims of sex-trafficking and exploitation, and it is by far and away men who are the vast majority of victimizers.