You are equating the happiness of the individuals involved as the basis for determining whether or not the action is moral. Its inherently a hedonistic perspective. Of course you can't make a moral claim that prostitution is wrong if you consider the "happiness" of the pimp, prostitute, and john as the highest good involved. Frankly, I'm not sure how you could make many moral claims considered reasonable based on that kind of criteria. Using the same measure you could justify all sorts of abusive situations so long as the exploited is "happy" with their circumstances.