As for facts, the one fact that is valid is that (like the previous poster said) consensual is the key. If you decide to sell your body , and an agreement is made, you are free to do so. When things get dangerous is when such "agreement" comes with strings attached, or when they are made with the wrong individuals who would end the "agreement" with blackmail, a fight, death, or any of those situations you see reported in the news sometimes when games go wrong.
There is also something to consider: Taxes. Believe it or not. If I go around making over 100,000 selling my body having sex all around, and someone reports that I am making all this money under the table, I can easily go to jail because it would be like like running any other business without a license. If by any chance I happened to carry a VD and give it to anyone whom I served, I could be also accused just like many women in the 1980's were accused of manslaughter for giving HIV to men, even after knowing that they had it (using your body as a deadly weapon).
So, there are lots of complications from these kinds of deals. As soon as you had a humans into a crazy idea, it gets crazier.
First of all, I assume you mean "without penalty." Second, there is no such thing as a fact here. This can only be opinion.
In my opinion, as long as the person truly is choosing to sell sexual favors, then that should be legal. If the person is somehow being forced to sell the favors (like by a pimp or something) that should remain illegal.
The only real reason why we keep this illegal is because of values that are supposed to come from our religion. However, many many people seem to violate those values all the time.
So, as long as it is truly consensual, I see no good reason to keep banning prostitution.
Attempting to legislate human behavior causes that behavior to increase, and if not legal, forces it underground. Humans are sexual beings, and therefore will express that drive, legal or not. What consenting adults choose to do in private is no one's business but theirs. There are always consequences to actions, and since all sexual activity incurs some element of risk, if such activity were legal, why not establish health standards for such who choose that type of employment?
Here is another opinion. First of all, if prostitution is illegal, then prostitutes are afraid to go to the police if a client robbs or beats them up. They would be arrested and fined for their illegal prostitution. They are thus victimized more often than people who are engaged in legal businesses. They often work for pimps so that the pimp can protect them, but the pimp often takes too much of what they earn or beat them up or force them to take clients that they do not want; he too knows that they cannot go to the police.
Having said all of that, I cannot say that it should be legal. (Here comes the opinion part.) Prostitution has an impact on the morals of the community, on the atmosphere of the community, on the children growing up in its proximity, on the type of people from outside of the community who visit within the community. The local community, in my opinion, should certainly have the right to ban or otherwise penalize or regulate prostitution. I do not think it is a matter for the nation-state; I think it is a matter for the local community.