Amongst our inclinations there is one which is directed towards other human beings...there is no way in which a human being can be made an object of indulgence for another except through sexual impulse. This is in the nature of a sense, which we can call the sixth sense; it is an appetite for another human being.
We say that a man loves someone when he has an inclination towards another person. If by this love we mean true human love, then it admits of no distinction between types of persons, or between young and old. But a love that springs merely from sexual impulse cannot be love at all, but only appetite. Human love is good will, affection, promoting the happiness of others and finding joy in their happiness. But it is clear that, when a person loves another purely from sexual desire, none of these factors enter into the love. Far from there being any concern for the happiness of the loved one, the lover, in order to satisfy his desire and still his appetite, may even plunge the loved one into the depths of misery. Sexual love makes of the loved person an object of appetite; as soon as that appetite has been stifled, the person is cast aside as one casts away a lemon which has been sucked dry.
Sexual love...taken by itself and for itself, it is nothing more than appetite. Taken by itself it is a degradation of human nature; for as soon as a person becomes an object of appetite for another, all motives of moral relationship cease to function, because as an object of appetite for another a person becomes a thing and can be treated and used as such by every one. This is the only case in which a human being is designed by nature as the object of another’s enjoyment. Sexual desire is at the root of it; and that is why we are ashamed of it, and why all strict moralists and those who had pretensions to be regarded as saints, sought to suppress and extirpate it. It is true that without it a man would be incomplete; he would rightly believe that he lacked the necessary organs, and this would make him imperfect as a human being; nonetheless men made pretense on this question and sought to suppress these inclinations because they degraded mankind.
Because sexuality is not an inclination which one human being has for another as such, but is an inclination for the sex of another, it is a principle of the degradation of human nature, in that it gives rise to the preference of one sex to the other, and to the dishonoring of that sex through the satisfaction of desire. The desire which a man has for a woman is not directed towards her because she is a human being, but because she is a woman; that she is a human being is of no concern to the man; only her sex is the object of his desires. Human nature is thus subordinated. Hence it comes that all men and women do their best to make not their human nature but their sex more alluring and direct their activities and lusts entirely towards sex. Human nature is thereby sacrificed to sex. If then a man wishes to satisfy his desire, and a woman hers, they stimulate each other’s desire; their inclinations meet, but their object is not human nature but sex, and each of them dishonors the human nature of the other. They make of humanity an instrument for the satisfaction of their lusts and inclinations, and dishonor it by placing it on a level with animal nature. Sexuality, therefore, exposes mankind to the danger of equality with the beasts.
But as man has this desire from nature, the question arises how far he can properly make use of it without to his manhood. How far may persons allow one of the opposite sex to satisfy his or her desire upon them? Can they sell themselves, or let themselves out on hire, or by some other contract allow use to be made of their sexual faculties?...
...Man cannot dispose over himself because he is not a thing; he is not his own property; to say that he is would be self-contradictory; for in so far as he is a person he is a Subject in whom the ownership of things can be vested, and if he were his own property, he would be a thing over which he could have ownership. But a person cannot be a property and so cannot be a thing which can be owned, for it is impossible to be a person and a thing, the proprietor and the property.
Accordingly, a man is not at his own disposal. He is not entitled to sell a limb, not even one of his teeth. But to allow one’s person for profit to be used by another for the satisfaction of sexual desire, to make of oneself an object of demand, is to dispose over oneself as over a thing and to make of oneself a thing on which another satisfies his appetite, just as he satisfies his hunger upon a steak. But since the inclination is directed towards one’s sex and not towards one’s humanity, it is clear that one thus partially sacrifices one’s humanity and thereby runs a moral risk.
Human beings are, therefore, not entitled to offer themselves, for profit, as things for the use of others in the satisfaction of their sexual propensities. In so doing they would run the risk of having their person used by all and sundry as an instrument for the satisfaction of inclination. This way of satisfying sexuality is prostitution, in which one satisfies the inclinations of others for gain. It is possible for either sex. To let one’s person out on hire and to surrender it to another for the satisfaction of his sexual desire in return for money is the depth of infamy. The underlying moral principle is that man is not his own property and cannot do with his body what he will. The body is part of the self; in its togetherness with the self it constitutes the person; a man cannot make of his person a thing, and this is exactly what happens in prostitution. This manner of satisfying sexual desire is, therefore, not permitted by the rules of morality.
But what of the second method, namely concubinage? Is this also inadmissible? In this case both persons satisfy their desire mutually and there is no idea of gain, but they serve each other only for the satisfaction of sexuality. There appears to be nothing unsuitable in this arrangement, but there is nevertheless one consideration which rules it out. Concubinage consists in one person surrendering to another only for the satisfaction of their sexual desire whilst retaining freedom and rights in other personal respects affecting welfare and happiness. But the person who so surrenders is used as a thing; the desire is still directed only towards sex and not towards the person as a human being. But it is obvious that to surrender part of oneself is to surrender the whole, because a human being is a unity. It is not possible to have the disposal of a part only of a person without having at the same time a right of disposal over the whole person, for each part of a person is integrally bound up with the whole. But concubinage does not give me a right of disposal over the whole person but only over a part, namely the sexual organs. It presupposes a contract. This contract deals only with the enjoyment of a part of the person and not with the entire circumstances of the person. Concubinage is certainly a contract, but it is one-sided; the rights of the two parties are not equal. But if in concubinage I enjoy a part of a person, I thereby enjoy the whole person yet by the terms of the arrangement I have not the rights over the whole person, but only over a part; I, therefore, make the person into a thing. For that reason this method of satisfying sexual desire is also not permitted by the rules of morality.
The sole condition on which we are free to make use of our sexual desire depends upon the right to dispose over the person as a whole–over the welfare and happiness and generally over all the circumstances of that person. If I have the right over the whole person, I have also the right over the part and so I have the right to use that person’s sexual organs for the satisfaction of sexual desire. But how am I to obtain these rights over the whole person? Only by giving that person the same rights over the whole of myself. This happens only in marriage. Matrimony is an agreement between two persons by which they grant each other equal reciprocal rights, each of them undertaking to surrender the whole of their person to the other with a complete right of disposal over it. We can now apprehend by reason how a sexual union is possible without degrading humanity and breaking the moral laws. Matrimony is the only condition in which use can be made of one’s sexuality. If one devotes one’s person to another, one devotes not only sex but the whole person; the two cannot be separated. If, then, one yields one’s person, body and soul, for good and ill and in every respect, so that the other has complete rights over it, and if the other does not similarly yield himself in return and does not extend in return the same rights and privileges, the arrangement is one-sided. But if I yield myself completely to another and obtain the person of the other in return, I win myself back; I have given myself up as the property of another, but in turn I take that other as my property, and so win myself back again in winning the person whose property I have become. In this way the two persons become a unity of will. Whatever good or ill, joy or sorrow befall either of them, the other will share in it. Thus sexuality leads to a union of human beings, and in that union alone its exercise is possible. This condition of the use of sexuality, which is only fulfilled in marriage, is a moral condition.
But let us pursue this aspect further and examine the case of a man who takes two wives. In such a case each wife would have but half a man, although she would be giving herself wholly and ought in consequence to be entitled to the whole man. To sum up: prostitution is ruled out on moral grounds; the same applies to concubinage; there only remains matrimony, and in matrimony polygamy is ruled out also for moral reasons; we, therefore, reach the conclusion that the only feasible arrangement is that of monogamous marriage. Only under that condition can I indulge my sexual faculties....
For Mr. Kant, having coitus with someone outside of monogamous marriage is bad because such an act renders a person an "object of appetite", and represents a "degradation of human nature." For Mr. Kant, then, the only solution, for sexual congress to be possible without degrading humanity nor breaking moral laws, is within monogamous marriage. Which seems perhaps to be an odd point of view, given that Mr. Kant was a lifelong bachelor.
For the Catholics, as described in an earlier post, a lustful look, in and of itself, whether inside or outside of marriage, is tantamount to adultery. For Mr. Kant, lust within a marriage would seem to be perfectly acceptable.
Modern Feminists often do participate in relationships that amount to de facto concubinage, and suffer no opprobrium. Many Feminists seek to satisfy their sexual appetites through casual sex, which is not morally superior to prostitution, although some Feminists would disagree. Most Feminists will find a lot of Mr. Kant's terminology appealing (e.g. "object of appetite", the assertion that persons are not things) because of its similarity to Feminist jargon concerning "sexual objectification." However, a lot of Feminists will reject his solution of refraining from sexual intercourse outside of monogamous marriage, as perhaps being outdated, old-fashioned and patriarchal.