One of the most pervasive while also one of the most ridiculous arguments in favor of certain presumed freedoms is the one about consenting adults. Simply put, it makes moral a given action on the grounds that the folks involved are in agreement as to their joint participation. It is ultimately a wholly untenable position.
To begin with, if all that is required to make something ethically acceptable is the agreement of the parties involved, then we really ought to allow duels. The (presumably) two participants agreed to take shots at each other. So no matter how barbaric, as they freely entered into the contract we would have to let them fire away.
Absurd. Yet when applied to other moral questions, and prostitution for example is one key area where the adage is often used, it is something accepted as axiomatic. The truth is that you would be on stronger ground simply to argue that paying for sexual favors is not in itself wrong rather than to say it's okay because there was no coercion.
The mere fact that people are willing to sell themselves that way while there are also folks willing to buy the, ahem, product, in fact reeks of coercive effects on its own standing. One person wants money, the other wants something which money can buy. A certain coercive effect is in fact at work. Still, even setting that point aside begs the question. The best answer is that nothing is good solely because those involved want to to do it. The act in question must be good on its own stead lest those involved be acting on mere impulse or selfishness. Or, indeed, actual immorality.
This is not to say that a free will act without pressure isn't a factor in moral decision making. It goes without saying that for most personal acts to be moral they must be entered into with a reasonable amount of freedom. In marriage, for example, both the man and the woman involved must do so of their own consent. Yet a hypothetical marriage between a man and a woman both of whom are free to marry is itself already moral. We are in fact beyond the issue of the morality of the potential nuptials by the time the question of will enters the fray.
In short, when considering the rightness of something there are two questions at hand. The first and most important query is whether the act is morally right, seen objectively, on its own. The second is whether those involved are the proper parties to it. Their consent is never a point until after the moral correctness of the action is assured.