Would legal prostitution be immoral?

This is a spinoff from this recent question, in which many believe prostitution is unquestionably immoral. It got me wondering exactly why it is considered immoral. The quick answer is for religious reasons, but if there is a woman who wants to provide the service without coercion, and a man who just wouldn’t be able to get any otherwise decide to have sex, why is this so bad? People with pent up sexual frustration and no outlet can do really bad, creepy things. On the flipside, I have to admit I would be horrified if my daughter went into such a line of work. So there is conflict in my mind about it. What do you think?