The same reasoning can be aplied to drugs.
The war on drugs is a failiure. It hasn't led to less addicts, only that life as an addict is more dangerous. If we admit that addicts are have a disease that need treating, not jail-time, then we're really in trouble. The war on drugs is making it harder for addicts to get help and leave their addiction, which hardly was it's goal. Since the war on drugs was initiated in the 70'ies, the quality of drugs has gone up many times. The reason for this is that with increased enforcement, profits for drug trade goes up and competition between dealers increases. The demand is the same.
Since one of my best childhood friends died from a drug overdose I've read up a lot on this. According to a bunch of Dutch and US studies the only effective thing we can do to help addicts and prevent people becoming addicted, (ie combat drugs) is to make drug clinics free and easily accesible. A junkie who wants to have drugs will always find them. What we need to do is make sure that the junkies who don't want to do drugs, can get all the help they need it getting off it. Stigmatising them as we do today isn't helping.
And lastly. There's two reasons people become addicted to anything. 1) genetic predispostion. 2) doing coke or heroin (or derivitives).
Exstacy, LSD, marijuana, alcohol etc have never in any studies ever been found to make people physically addicted. You need to have the genetic predisposition to become addicted to these, and it's a minority who are likely to carry this gene. You need two short serotonin or dopamin receptors in the ganglia or you haven't got it. These people can become addicted to anything. anything. Not necesarily a substance.
Drug enforcement is today an industry and very many people make their living off "fighting" drugs the old Nixon hard-line way. Science tells us that the only way to combat drugs is harm reduction. Any other aproach is pure religion.
Just as making drugs illegal only leads to more damage, illegal prostitution leads to more dangerous lives for prostitutes.
And on an end note. Just because we make something legal doesn't mean we remove all regulation from it.
edit: and to be perfectly clear. I'm not a drug liberal. I don't want a society where everybody can do drugs if they want to. I want a world with less drugs. And I want a world where nobody is forced into prostitution. I also believe that we should let science guide us into making this come true.