In the Metaphysics of Morals , Kant defines sex as “the mutual use which one human being makes of the sexual organs and faculty of another.” This mutual use aims at pleasure. He acknowledges that in using the sexual organs of another, one is acquiring use of the whole person, since “the person is an absolute unity.”
At the same time, sexual intercourse represents a profound alienation: “In this act, a human being makes himself into a thing, which is contrary to the right of human nature to one’s own person. This is possible only under one single condition: when a person is acquired by another in a manner equal to a thing, correspondingly the former acquires the latter, for in this way the person gains itself back again and reconstitutes its personhood . . . . this personal right is nevertheless at the same time also a right in the manner of a thing,” and this is clear since “when one part of the couple has run away or has given itself into the possession of another, the other spouse has the right at any time and without any condition to take it back into his or her power like a thing.”
Sex is not self -gift, but only the gift of sexual organs for use; it is self-alienation, not self-gift. A sexual ethic more deeply formed by modernity is hard to imagine.