Consent is our rough-and-ready standard for moral sexual activity, but it’s pretty easy to see that the standard will break down in a number of non-typical cases, and that we’ll ultimately have to abandon it as a formal criterion for moral goodness in all cases. Ultimately, we’ll have to abandon it not just as a moral criterion but even a (sufficient) legal one.
The consenting adults criterion is just Thrasymachus with a democratic spin. All relationships between persons fall under justice, and so one simply replaces the claim that justice is the will of the stronger with justice is the mutual will of equals. So taken, it will have a very hard time getting past critique that Plato gave of it. One has to take any act of will either as it is made by actual persons or according to some idealized standard. If we take it in the first sense, then the one willing can be deceived, mistaken, or perverse, and such willing need not be something good or just for the one willing; if we take it in the second sense then our actual exercise of will derives its justice from an idealized, extrinsic account of the moral life which is yet to be determined. In spite of being proposed as a standard for what will count as moral sex, any way of understanding the consenting-adults standard leaves us with no standard for what will count as moral.
The simpler critique is to point out that thinking things become good because we want them is like thinking things become triangular because we see them. Goods are experienced as given, even when they admit of the variety of tastes.
But maybe the consenting adults criterion isn’t an attempt to target the just or the moral but the legal. Maybe the point is not to figure out what will count as wicked or virtuous but simply what will put one in jeopardy of prison and fines. The idea seems to be that the law will only be cognizant of rape and let everything else pass. Even then consent is a problematic criterion. Absence of consent is an identifier of rape, but consent is not an identifier of the absence of rape (cf. this also). If rape is just sex in the absence of consent, then the law will have to deal with consent as it occurs in actual people, and this is a messy affair. Consent that is totally (or sufficiently) uninformed cannot count as consent, no matter how enthusiastically it might be given. There is an additional problem that the consent criterion tends to prejudice the law in favor of the accused, and it’s far from clear that this is the best way to ensure justice in rape cases.
The deeper problem, however, is that rape is a violation of justice whereas perverse sex is a violation of temperance. If the only cognizance the law can take of sex is so far as it is tied up with rape, we’re effectively saying that the law has nothing to say about sexual activity at all, in spite of the enormous effects that sex has on our lives. This is an insupportable and ridiculous position which will resolve itself eventually into some legal declaration of what will count as acceptable sexual activity and what won’t – and mutual consent is not gonna cut it. In fact, the law will probably be forced into defining what will count as chaste precisely in response to the inadequacies of the consenting-adults criterion.*
*We’ll probably go through a phase where we attempt to judge sexual relations by contract-law standards, but this will only highlight the intrinsic absurdities of the consenting adults criterion. “Consent” will mean the filling out of the proper forms. At this point, we will have shifted from merely not solving the problem to failure even to understand the question. However perversely we’ve understood the physical act of love, we’ve never sunk so deep as to understand it as bureaucratic.