Here are some facts about sex and consent.
- Without valid consent, sex is always seriously wrong.
- Merely proxy consent for sex (say, by parents on behalf of a child) is never valid.
- Child consent for sex is never valid.
- Consent may be withdrawn at any time when discontinuation is still possible.
- Sometimes sex is permissible (with consent, of course).
There aren't many cases other than sex where the analogues of 1-5 apply. Here's one potential such case. The Nuremberg code forbids medical experiments that involve a significant risk of serious injury or death to a healthy subject, except in the case of self-experimentation, assuming the other conditions of the code are met. But if such self-experimentation is permissible, it seems likely that it would be morally permissible (though we may have good reasons to rule it out in professional medical ethics codes) to hire someone to perform such experiments on one. To do such experiments without valid consent from the employer/subject would be seriously wrong, neither proxy nor child consent would be sufficient for validity, and one would have to stop whenever consent was withdrawn.
But notice an important feature of the medical experiment case: the reason these strong consent restrictions are in play is because of the significant risk of serious injury or death. If one modifies the experiment to make the risk insignificant, weaker consent standards come into play. In particular, parents will then be validly able to consent.
But in the case of sex, the reason for the strong consent standards does not come from risks of injury or death, whether physical or psychological. For we can suppose a case where the person is unconscious, where 100% effective prophylactics are used and where the person will never be informed of the event, and hence there is no danger of physical or psychological injury. Even so, the strong consent standards for sex apply. For instance, merely proxy consent is still not sufficient.
Notice, too, another interesting feature of the medical experiment case. Even when the experiments are done in a moral good way, it is regrettable that there was no other way of getting the benefits. But sex isn't like that: when it is engaged in in a morally good way, typically there is nothing regrettable about it--quite the opposite.
So there seems to be something exceptional about sex and consent. The other cases where such strong requirements of consent need to be in place are look to be cases where one needs permission to impose something very bad on someone. That's not what's going on in sex. What is going on? My view is that sex is tied very closely to love, and love requires freedom... But filling out detail isn't easy.