I don't know, and that's a good question. If it's two consensual adults, I see no reason for other people to stick their nose in their business. Just because it's something that I wouldn't do, doesn't mean that I have any right to tell someone that they can't.
Because prostitution is often happening in a grey area that is connected with crime. Drugs, human trafficking and such things. I dont believe that making it illegal does help to stop that, though. Here in Germany it was made legal, to give the prostitutes the chance to claim their rights and insist on certain standards, like any other worker can. It has brought some improvement, but some problems still remain.
Most of the answers deal with prostitution as an illegal activity, getting beat up, human slavery, etc If it was legal (and where it is legal) houses are clean, workers are given regular checkups, violence is prosecuted and workers pay taxes. I say legalize it.
Because when the government can control something with the people's blessings, they will. It also brings in revenue just like speeding tickets. Personally I wish Congress would just stay home and meet once a year to pass a budget. For every law passed is a freedom lost (for somebody)
It could be illegal because of immorality and because many women are forced into it but I don't think it should be illegal because it could be consenting and what people do in private is their business and not the governments. In Nevada prostitution is legal and regulated. If it is regulated and many people are checked for stds regularly like in Nevada it should be legal.