Legal way to be a prostitute?

First off no I am not thinking of becoming one. We have had some shady "massage parlors" get busted recently in town and it made me think obviously having someone get a massage and pay extra for a happy ending is illegal but what if you charged a large sum for a massage and even had the patron sign a contract saying what they are paying for is strictly a massage but that if things get heated and both parties are willing sex acts may occur but are not paid of the paid massage but rather a decision to engage in sex acts between 2 consenting adults? How would that work out legally? Is their a loophole there?