Why is prostitution illegal? I can think of 3 main "reasons", although I am in favor of legalizing it. The main reasons are:
1) As a moral issue, it is a terrible "vice", it is considered sinful and offensive to public morality by a lot of people(very often for religious reasons). It somehow "cheapens" sex, and is harmful to "family values" and the "social fabric".
2) As a public health issue, many people think it helps spread various diseases.
3) As an issue of civil/women's rights, it is thought to be exploitive of poor, uneducated, abused, sometimes drug addicted women who can't find alternative employment that pays as well. Prostitution is like a "trap" for at least some prostitutes, and if alternative employment paying good money was available to them, they'd leave prostitution behind at the drop of a used condom.
Now as a person who favors legalizing prostitution, I can't really sympathize with the first reason so long as they are consenting adults.
As for the second reason, if in fact prostitution does play a huge role in spreading disease in society, then perhaps it should be illegal or at least very closely monitored by the authorities if it is legal.
The 3rd reason may be the most serious. A lot of young women desperate for money get involved in prostitution when they may be too young to know any better. They often come from poor, abusive, broken homes and have difficulty finding permanent employment. Sadly, many prostitutes start their "work" even before they reach adulthood. And of course, there are many prostitutes who are forced into prostitution. Some of them even suffer from serious mental illness. Even as an atheist, to me there is something a little "icky" about prostitution, especially the thought about it being a "trap" for poor women.
I find myself basically agreeing with Cyborg about this "exploitation" issue being used as a red herring by those who favor keeping prostitution illegal. Or rather, they "misuse" it as a red herring since they don't understand how legal prostitution can help lead to less exploitation. Now since I am in favor of legalizing it, I am not so much using this as a red herring tactic so much as I am simply trying to illustrate why so many people believe prostitution should be illegal, and that even a person like me who favors legalizing it finds himself sympathizing with the women in this "trap". I don't think pointing out that some kind of serious exploitation is taking place should be dismissed outright, as if it is "peachy keen", even if it is not central to the issue of prostitution's legality. That is the main reason I'll probably never do any business with a prostitute, although I don't believe I should have any "right" to stop anyone else from going to one(so long as they are consenting adults). The poorer the women are, the more likely they are to be exploited, generally speaking, and the more it seems likely we are in a "gray" area legally speaking - perhaps they started before they became adults for instance, among other things. The point should be driven home again and again to those who want prostitution to remain illegal due to "exploitation" - that if prostitution is legal, it could help prevent exploitation and the worst abuses, since they will be under police protection instead of being looked at and treated as the "criminals" they are now.
I believe that legal prostitution should be between consenting adults only, 18 and up. There should be very serious penalties for forced prostitution or pimps employing underage girls(just about all the laws regarding this already exist). Legalizing prostitution might even make prostitution safer. Prostitutes should be licensed and registered with the city or state and given regular health check-ups(like they do in the Netherlands), and customers should be required to use protection. They should be confined to Red Light districts and away from schools, and should have police protection. Prostitutes with drug problems should have easy access to counseling and detox. The legal authorities can then focus their attention on illegal prostitution. Truth be told, rape, sexual abuse, exploitation, underage sex, human-trafficking, theft and slavery are already illegal, as has been pointed out before, so no new laws may be required to help stamp out illegal(forced/underage) prostitution.
Just my 2 cents.