The theory behind decriminalization of prostitution is that it avoids the problem of dragging women into the legal system and helps avoid other related crimes. Indeed, a few years ago one such report found a decline in reported rapes in Rhode Island (where it was legal at the time)
The decline in the number of rapes was so large that Cunningham and Shah felt obliged to examine their data with three separate statistical methods, but the effect persisted. The authors were eventually persuaded that their result was not a fluke, and that imposing criminal sanctions on prostitutes and their clients might cause violence against women. "The human costs are so big, if this is in fact a very real causal effect," Cunningham said. "I think we have convinced ourselves that we have done everything we can do rule out alternative explanations."
The problem with legalized prostitution, however, is it tends to run hand-in-hand with a far more serious (and much less accepted practice): human trafficking (a close cousin of sex slavery). It's well known that most prostitutes have a boss, known colloquially as a pimp, who take most of their earnings. But what's less known is that many pimps "groom" the women under them via rape, after having kidnapped them. They use their status as prostitutes against the women (sometimes girls) to convince them to remain in their profession. Indeed, one charge levied against legalized prostitution is that it makes prosecuting human trafficking difficult, if not impossible
Rhode Island police are stymied by the lack of laws enabling them to investigate effectively in order to identify victims and prosecute traffickers. Prostitution was decriminalized in the state as a result of a lawsuit brought by a prostitutes’ rights group in 1980. Since then, Rhode Island has had no laws against prostitution so long as no solicitation occurs outdoors. Outdoor solicitation remains illegal as a form of loitering for indecent purposes. Rhode Island has laws against sex trafficking and pimping (pandering, transporting, and harboring for prostitution, and deriving support and maintenance from prostitution). But without a predicate prostitution crime, state police lack the grounds to intervene and interview likely victims. Enforcement of federal sex-trafficking laws is also severely hampered. Consequently, there have been no federal or state prosecutions for sex trafficking and no state prosecutions for pimping for many years.
There's not an easy answer here. If you focus laws against "Johns" (prostitution clients) you run the risk of human trafficking running unabated. If you make the prostitute herself a criminal, you run the risk that she herself is a victim, forced into a world she did not choose. Either way, prostitution is not a victim-less crime, and any laws need to protect women first.