Absolutely, it should be. Regulate and restrict it, as others have said, but it should still be legal.
There is a reason that it is referred to as the oldest profession. Nothing will make prostitution go away.
And it is not right to tell anyone that they can't profit from having sex with others when they can give it away for free with the same exact chances of spreading disease. Not only that, but porn stars are paid to have sex on camera, so I don't see why other women and men shouldn't be able to just be paid to have sex.