I don’t think it follows that something should be outlawed because it is immoral. I can think of many things that I believe to be immoral (such as hypocrisy, greed, pay-day loan operations) that I don’t think should be outlawed.
I think morality is a very complex topic that spans both the objective and the subjective; there are things (stealing, murder) that everyone agrees are immoral and other things (booze, gambling, pot, abortion) that aren’t quite as clear cut.
I think that prostitution falls in to the subjective category: while it’s true that people from all the political camps are for toughening prostitution laws, it doesn’t follow that a majority of people in all political camps are for toughening prostitution laws. For example, you could find “evangelicals, feminists, liberal activists and conservative human rights advocates” whose favorite color is blue, but that doesn’t mean that there’s universal agreement that blue is the best color.
Because we’re a country with such a diverse set of beliefs, I think the criteria used to determine if an act should be outlawed needs to be more complex then just “it’s immoral.” I think we also need to look at the impact the act in question has on the common good, the impact that criminalizing the act will have on those who practice it, and we should also consider if it’s possible to regulate the act without criminalizing it outright (an in doing so strike a balance between the two).
I, personally, think that prostitution is immoral. And I think that prostitution, as it currently exists in most of the United States, does harm the common good, because it harms the health and well-being of poor desperate people, brings down property values, is often accompanied by other crime, etc.
But I don’t think it has to be so: I think regulated prostitution, like what exists in parts of Nevada, seems to do a pretty good job of reducing the harm prostitution does to society and the sex workers and at the same time gives people who want to participate in prostitution the legal oppurtunity to do so.
Less verbosely: Prostitution is immoral; laws making prostitution illegal are just; laws making prostitution legal but highly regulated are also just; the exact nature of prostitution laws should be up to individual locales. Nevada has it right.