One vague but real division line among thinkers about sexuality is whether sexual cases are in some important way different from all or most other cases. Some writers on the right, for instance, think that in sexual cases categories like "the sacred" or "desecration" are applicable, or that there are higher moral standards in sexual cases (so that, e.g., it may not be immoral to misuse a finger, but is wrong to misuse a sexual organ, or it is not always wrong to use someone non-sexually but always wrong to use someone sexually). At the same time, the idea of sexual cases as somehow different has historically also been found among some left-leaning folk as well, such as in quasi-religious ideas of transforming the world through removing our culture's sexual restrictions (one probably wouldn't talk of transforming the world through removing our culture's dietary restrictions[note 1]), or in the idea that sexual oppression is particularly bad. Other writers, on the other hand, hold that sexual ethics is not in any significant way different from other areas of ethics (C. S. Lewis says this explicitly in The Four Loves).
Who is right? Well, here I just want to note one way in which our attitudes towards sex are different from our attitudes towards other activities. Sex is impermissible without consent, and while there are other activities of which that is true, the requirement of consent in sexual cases is much more stringent than in most non-sexual cases. Here is one way in which this is so. For some activities, such as the eating of ice cream, the consent of a minor or someone generally incompetent is acceptable. For many activities for which the consent of of a minor or someone generally incompetent is acceptable, such as medical procedures, the consent of a proxy is sufficient. However, in sex we are suspicious of the consent of a minor and we do not allow proxy consent. There are not many other cases like that.
That is not to say that there are no other cases like that. Some Christian denominations that reject infant baptism can be seen as treating baptism in this way. I can also see how someone might take this view of certain kinds of major life-changing medical procedures that arguably do not treat an organic condition, like physician-assisted suicide (here "life-changing" is an understatement), sex-reassignment or the amputation of the limb of an apotemnophiliac. Note that an analogy between sex and these cases underscores the idea that there is something momentous about sexual cases. Our rules on voting are somewhat similar but not quite the same: one must cast one's vote oneself, not have a proxy cast it for one[note 2]and one must be of age, but the difference is that one does not need to be competent other than by age.
If there is a difference, we may ask why there is such a difference. One consideration is that sex is a momentous matter because it is closely related to life-and-death matters—sex does, after all, involve the functioning of reproductive organs (this is true not just in the case of intercourse). Another is that love is always something momentous—the duty to love is the ground of all other moral rules—and sex ought to be the consummation of a particular kind of love (eros), so it inherits momentousness from it; moreover, one might argue that the particular kind love that sex ought to be the consummation of a love between free and equal persons, and consent thus is plausibly required. One might also bring in contingent psychological features of sexual cases, but I would prefer not to do that, because those could be absent, and the consent requirements would still be in place. In any case, it is clearer that there is something different about sex than most other activities—that is a datum—but what that difference is is harder to capture.