Kant never married, there is no evidence that he was romantically involved with anybody and his ethical writings contain few direct references to sex. Because of these well-known biographical details, it is unreasonable to suppose that Kant would struggle when it comes to sexual ethics. Yet, as this essay will argue, his approach to ethics is far from being too abstract to be helpful when making decisions about sex.

Firstly, Kantian Ethics are of clear relevance to making decisions about sex because they concern how we choose to treat people. Kant argued that reason demands that we treat human beings, whether in the person of ourselves or another, always as an end and never as a means to an end. For Kant, human beings are “pathologically loving”, recognising that it is rational to treat other people as we would wish to be treated. This means that in any moral situation – and sex is most definitely a moral situation because it affects human wellbeing like little else – we must consider and protect the interests of all persons equally. This rules out using somebody for pleasure, whether through heterosexual or homosexual intercourse, and only allows sexual activity that is consensual and supportive of both parties’ long-term wellbeing. Marriage would be the obvious (though not necessarily the only) way to ensure properly informed and enthusiastic consent, including as regards possible children, as well as mutual commitment to the others’ wellbeing. Adultery would be ruled out by the impossibility of universalising breaking promises. Casual sex could be as unacceptable as rape, because it is probably underpinned by the same un-universalisable maxim. This means that Kantian Ethics would helpfully reinforce common norms of behaviour, supporting marriage and discouraging adultery, promiscuity and of course sexual abuse and violence. On the other hand, for Kant there is no essential moral difference between heterosexual and homosexual sex, meaning that Kantian Ethics could be more useful than Natural Law in the 21st Century.

Secondly, Kantian Ethics are far more helpful than is Utilitarianism when it comes to making decisions about sex. Act Utilitarianism demands that decisions are made situationally relative to the predicted consequences. Nevertheless, as even the utilitarian Peter Singer admits, it is often not possible to predict consequences accurately. Also, making an objective decision when affected by lust is impractical. As St Augustine rightly observed, lust makes us incapable of doing what we know we should do. It also makes us lie to ourselves to get what we want. For example, if somebody was making a utilitarian calculation about having a one-night-stand, they start by making the assumption that this is a one-night-stand (which might not be the case) and then attempt to calculate their own feelings and that of the other party during and after sex. Quite obviously, these calculations might be inaccurate. Can they know whether they, or the other party, has an STI or mental health condition? Can they know that no conception will occur? Even where extensive discussion has taken place, the facts may turn out to be other than was thought. Further, such detailed discussions are not always practical in the real world. It follows that Act Utilitarianism is not really very helpful when it comes to making decisions about sexual ethics, only encouraging to individuals in pursuing their selfish pleasure. Further, Rule Utilitarianism is little improvement over Act Utilitarianism in practice. Few Rule Utilitarians propose imposing absolute rules other than perhaps “do not murder,” so people are permitted to break such rules as exist when they don’t seem useful. When it comes to sex, it is all too easy to see one’s situation as exceptional, leading Rule Utilitarians to become Act Utilitarians when it comes to sex. The exception might be John Stuart Mill, who famously kept his relationship with the married Harriet Taylor platonic, even though she was separated from her husband and the world assumed her to be his mistress. Mill respected the institution of marriage on utilitarian grounds, placing the happiness of society ahead of his own, and Harriet’s, pleasure. Yet would his utilitarian decision have been the same today, with easy divorce and different sexual mores? Utilitarians have to make decisions relative to the situation as it is, including social attitudes and laws, and today neither the law nor social attitudes impinge so much on individual sexual ethics as was the case in the mid 19th Century. It follows that today Kantian Ethics offers a more helpful guide than Utilitarianism when it comes to sexual ethics, because it reminds people to consider every person as an end and to act on universal principles rather than to give in to lust.

Of course, Kantian Ethics has its weaknesses. Some Utilitarians will suggest that Kantian Ethics rules out consensual promiscuous behaviour, which has the potential to produce a great deal of pleasure. As an absolutist system Kantian Ethics imposes general rules which reduce legitimate opportunities for happiness which might be allowed by a more flexible consequentialist approach. In addition, arguably Kant’s concern for reason controlling the animal instincts and for the damaging effects of making selfish decisions even once might rule out using pornography, even that which is computer-generated. It might also rule out masturbation. Again, Utilitarians would criticise Kant for this, suggesting that his absolute rules have reduced net pleasure unnecessarily. Nevertheless, it is Kant’s difficulty with the institution of marriage that presents a bigger problem to the usefulness of his ethic today. As Christine Korsgaard has observed, there is a potential issue with marriage for Kant, both because of the potential of the whole institution for using women as a means to an end and because of what it actually consists in. If marriage is, as it has long been, an instrument for the legal subjugation of women then no Kantian could allow that a woman could freely AND rationally agree to it and, if the woman did not agree both freely AND rationally, no man could freely AND rationally agree to it either. It is not possible to universalise agreeing to a contract which has either been forced on or not been understood by the other party; to do so would surely use them as a means to an end? Further, could a Kantian choose to marry when marriage represents an unbreakable promise or contract in the words… “Immanuel, will you take Christine to be your wife? Will you love her, comfort her, honour and protect her, and, forsaking all others, be faithful to her as long as you both shall live?” The implication of this wording could be that each partner promises to put the interests of the other partner first, even ahead of their own interests. Could such a promise be made freely AND rationally – or would entering into such a promise bar one from having a good will, which requires that all persons are treated strictly equally and not preferred on any grounds of personal preference, relationship… or presumably legal pre-contract AKA marriage? Korsgaard suggests that these issues can be overcome in the 21st century because legal obstacles to marriage being between equal partners have been dissolved and because the wording of the marriage service need not be interpreted – or even spoken – in this way. As Marcia Baron suggested, marriage-partners need not agree to prefer each other morally and in fact as rational and free people would resist any idea that they should do so. Nevertheless, using the sort of extreme thought-experiment beloved of Kant in the Groundwork, imagine that a newlywed couple is caught in a hotel fire. The bride escapes out of the third-floor window, maybe abseiling to the ground using her cathedral-train, and has the choice of helping her husband to make a safe descent or leaving him hanging as smoke billows from their window to run to reception and raise the alarm for the other guests. Who would think that her promise to love and comfort him did not cover such situations or that she would be justified in abandoning him to fate, provided that she did her duty by unknown others? WD Ross, in many ways influenced by Kant, argued that people have a prima facie duty to family members – including husbands or wives – but like Kant offered little clear guidance on how to resolve clashing duties beyond suggesting (again like Kant) that rational intuition should be our guide. This is the biggest difficulty with applying Kantian Ethics to issues arising from sex – that clashing duties are common and that Kant is not particularly helpful when it comes to helping people to resolve them. Saying that negative duties always take precedence over positive ones is not convincing or useful when family-members are concerned. Would anybody in the real world allow their wife or baby to starve rather than steal a loaf of bread and still have any expectation of having their good will rewarded?

In conclusion, despite being abstract, Kantian Ethics are more helpful than alternatives such as Utilitarianism and Natural Law when it comes to sexual ethics. In particular, Kantian Ethics is useful in encouraging people to focus on treating people as ends and not as means to an end. However, there are still significant problems with Kantian Ethics and the guidance it offers, particularly when it comes to how to resolve clashing duties, and these difficulties are not reserved to sexual ethics, but beset the application of Kantian Ethics more generally.