Because it’s just that – your own version. How can you make a *legal *prohibition on a purely moral basis when you admit that other sane humans might have a different version? Just because a lot of people agree doesn’t mean you’re all right. Nor is that the point, in my opinion.
I know the Supreme Court doesn’t agree with me and instead believes that the police power is virtually unlimited – the government can legislate to safeguard the health, safety, welfare and morals of the public. I think that last one ought to be left out.
The problem is that a law which prohibits a “victimless crime” (or, to put it a little more accurately, a “crime” in which the perpetrators *are *the victims) like prostitution, gambling, drug use, jaywalking, driving without a seatbelt, etc. simply cannot be justified on social contract grounds, which most people agree is supposed to be the theoretical basis of the legitimacy of governmental authority in a democracy.
What I mean is that most legal prohibitions can be viewed as vast, multiparty (implied) contracts in which all of the parties have contracts with all of the other parties, and the terms of the contract are simply promises to everyone else that you won’t do X. By way of example, consider the crime of larceny (theft.) I promise you I won’t steal your stuff, and you promise me likewise. We have a contract. And there is consideration flowing both ways – your promise not to steal my stuff has value to me, and vice versa.
By contrast, consider your example, prostitution. How would my promise not to engage in prostitution have any value to you? I certainly don’t care whether you do it. There is no consideration to support this deal. Why, then, would I want to invoke the machinery of the state to keep you from doing something you may want to do?
The response “it’s wrong” just shows a lack of respect for those who don’t agree. Obviously there are people who don’t think it’s ethically wrong. Who are we to stand in judgment of them?
I think my social contract argument above, if fleshed out, would provide a reason why all legal prohibitions ought to be antipaternalist – that is, the only legitimate basis for a legal prohibition is that it forbids people from doing something which involves unconsented-to harm to another. Personally, I simply take this principle as a given. A society which purports to tell the minority what is good for them is, to me, so arrogant, condescending, and unjust. Sorry, you can see this topic gets me a little worked up.
As an aside, I know I’m painting with a very broad brush in the examples of “victimless crimes” given above. I know there are at least facially reasonable arguments that in each case, there is a non-paternalistic justification for the law (with the possible exception of gambling – I just can’t see any even arguably legitimate reason to outlaw gambling.) I include them as examples because the justifications given for these laws are almost always paternalistic (e.g. you shouldn’t do drugs because it’s bad for you.)
Further reading: John Stuart Mill, “On Liberty.” Full text available here.