Kant never married, there is no evidence that he was romantically involved with anybody and his ethical writings contain few direct references to sex.  Nevertheless, his approach to ethics is of clear relevance to making decisions about sex because it concerns how we choose to treat persons, both ourselves and others and because in a way sex is the issue most likely to affect persons and their welfare in the long term.

In terms of deciding whether to have sex, Kant’s system would suggest that the decision should be a free and rational one if it is to be good.  Like Plato, Kant argued that human beings exist on three levels – our animal instincts (bestimmung), social and emotional selves (willkur) and our rationality (wille).  Like Plato, he argued that reason should be in command of desires emanating from instinct, such as lust, or from social and emotional concerns, such as friendship, what other people think etc.  The individual moral character (willkur) should come to reflect universal reason (wille) and not be tainted by selfish instinct in any way. When deciding rationally about sex it must be clear that the welfare of another human being is at stake, as well as your own welfare in the long-term… and in a heterosexual relationship, that of potential persons as well.

The Categorical Imperative demands that we consider the interests of all these persons as ends in themselves and choose to do only what can be universalised and set up as an example to others.  In practice it seems that the Kantian could not justify sex outside the context of a long-term committed relationship, because of the potential for people to be used as means to an end outside of that. Kantian Ethics seems likely to oppose adultery because it depends on deceiving people, along with casual one-night stands and rape of course… but arguably there is no barrier to homosexual sex, provided that it takes place within a committed relationship.

The application of Kantian Ethics in this way shows that it is a useful approach to decision-making, both in that it is practical, offering guidance that people can understand and accept, and in that it is likely to lead to more sustainable happiness than consequentialist ethics.

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.

Other Utilitarians are likely to agree with Kant that certain rules are necessary particularly when it comes to sex, because of the potential for young and vulnerable people being hurt.  For them it would be a matter of weighing up the potential pleasure in sexual morality being only self-regulated against the potential harm done to or by those who make bad decisions as well as their potential children.  For Kant the principle would be far more important than any consequential calculation – can we accept that behaviours which allow people to indulge their animal instincts, often by using other people as a means to an end of achieving sexual satisfaction, should be lawfully part of a “Kingdom of Ends”…  Kant would say not!  Either way, the need for moral rules in relation to sexual issues is clear.  The short-term physical pleasure that might be produced through consensual promiscuity is more than outweighed by the long-term harms that allowing it is likely to cause. Further, a consequentialist approach to sexual-decision-making is particularly problematic because of the difficulty of working out what activities are likely to cause what quantity and quality of pleasure to other people – who is really a competent judge in these matters? – and because of the high probability of unintended consequences.  An act-utilitarian approach to sexual ethics seems less practical and less likely to produce happiness either in terms of quantity or quality – than a rule-based approach.

Given that Kant seems to support sexual activity only within a long-term committed relationship, perhaps could only be justified within marriage?  Yet marriage raises an interesting question for the Kantian.  As Christine Korsgaard has observed, there is a potential issue with marriage 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 21^st 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?