The original, practical reason prostitution was illegal was to ensure population growth. Men who visit prostitutes waste both money and sexual energy that could otherwise be spent impregnating a wife, having and raising kids.
But this was the original reason there were laws against sodomy and homosexual marriage too, laws which have in the past few years been abandoned. I think even pornography used to be illegal for similar reasons. The government now even pays for multiple forms of contraception, so women are free to have sex with little interest in kids.
Since the government no longer seems to have any interest in ensuring population growth, it makes little sense why prostitution is still illegal. Homosexuals and women get theirs, while heterosexual men suffer.
But to play devil's advocate, if prostitution should be illegal, then all that other garbage should be illegal too. Make contraception and abortion illegal, so women will have to make responsible dating choices and stop acting like sluts. Make porn illegal so we don't end up with a bunch of degenerate men who don't know how to interact with real girls. Make homosexual marriage illegal, so our children are no longer subject to their aberrant, abhorrent influence.
Pick one. I'm just sick of living in a society where I'm surrounded by sex but can't have sex. Imagine if you were surrounded by food and told you couldn't eat it. You might eventually get very angry.