Sexual union (commercium sexuale) is the reciprocal use that one human being makes of the sexual organs and capacities of another (usus membrorum et facultatum sexualium alterius). This is either a natural use (by which procreation of a being of the same kind is possible) or an unnatural use, and unnatural use takes place either with a person of the same sex or with an animal of a nonhuman species. Since such transgressions of laws, called unnatural (crimina carnis contra naturam) or also unmentionable vices, do wrong to humanity in our own person, there are no limitations or exceptions whatsoever that can save them from being repudiated completely.
(Immanuel Kant, The Metaphysics of Morals, trans. and ed. Mary Gregor, Cambridge Texts in the History of Philosophy, ed. Karl Ameriks and Desmond M. Clarke [Cambridge: Cambridge University Press, 1996], 61-2 [first published in 1797] [italics in original])
