"Harm" does not have the same definition as "offend", but you seem to constantly mix them up. You want to prohibit prostitution because it offends you. But if you want to justify a ban on prostitution, you have to come up with some examples of how it harms you.
Offending a person can be called a kind of harm, because it makes tham unhappy and lowers the quality of life if its allowed all the time. Offendign people can surely LEAD to harm if it starts violence.
...it's perfectly legal in the USA to create pornography, publish racist hate literature, or declare that you don't think god exists.
Those are what a lot of peole call "harmful IDEAS"... if you think race hate literature should be legal, just pray I never catch you, white buggahman! Godfearing types who believe everyone should know the word of the Lord do definitly think that denying His existance is harmful, to everyone.
That said, I do think prostitution should be legal. Its only unsafe and dirty because the law keeps it under ground. If hookers were a legit trade and could seel their punaany legally, thered be no violent pimps or punters getting away with beating them up, theyd get better funded protection against VDs and pregnancy, and less crime bosses getting rich off whorehouse profits. Like Pjdude said.