Using the space to dump an essey in it, wall of text warning.
In the seminary it was apparent that Kant's view on sexuality was generally seen as outdated and without justification in modern world. In a sense, it does fail to prove, that moral sex is possible. Kant's ethics are just too strict for it to give sensible basis to moral sex. How exactly, I will show in my essay. I will be (at least partly) operating using Kant's moral theory as basis to show it's inner conflicts. Second part of the essay deals with the reasons, why Kant's theory has relevance. The essay is in English, because I'm also writing it to someone else, I hope it isn't an inconvenience.
I Critique
In Kant's view sex is natures method of procreating. The agents, including humans, however do it for the purpose of pleasure and therefor use another person as a means not an ends. It conflicts with the basic notion of Kant's ethics to always see other people as ends, not as means. To sneak past that, the only solution for him was giving your all to another person and by receiving the same from the other person, “winning yourself back”. And the only way of achieving that, was through the institution of marriage. The argumentation seems somewhat justified at first, until one notices a striking resemblance. It is exactly the same logic as Rousseau uses in his definition of (positive) freedom: by giving all your freedom to general will / society / other humans, you gain back just as much and in the end everyone is completely free. And lets not forget, that people should be forced to be free. If the analogy fits, doesn't it make a hidden assumption, that people should be coerced to marriage? As Rousseau's definition has proven to be of little to do with practice of freedom, Kant's view on marriage is in no way changing the nature of sex. It just serves to confuse people by claiming that you gain a right or a license to use your partners body from it. If a marriage was indeed such union of will as Kant described, then a married couple would also have an equally strong right to beat each other senseless. Practical imperative would forbid giving a right to beat someone senseless and, on the same basis, sex too. There for I think I can conclude that marriage is not a miracle cure to the problem morality of sex. Kant had serious trouble transforming sex from animal instinct to human nature. It is an impossible task, or rather an unnecessary one as I will go on to demonstrate.
Previous passage leads to another problem - the questionable value of the claim, that people give up their humanity if they have consensual sex. Is sex in itself something that is possible to regulate through morals or is it a basic need for all animals, much like food and sleep? For Kant our difference from animals is important and sex “exposes mankind to the danger of equality with the beasts”. Even though nature designed humans to become objects of sexual desire, they should avoid it. However avoiding sex is quite hard and if you apply categorical imperative for the fun of it, the human race would die out in 100 years for the sake of being moral (Though I should note, there might be a workaround to avoid extinction using biotechnology – basically growing children in jars - but I'll leave that unattended for now). Obviously that makes the practice of Kant's view on sex undesirable and frankly, impossible. Either giving up your humanity or dieing as a species, no matter how you look at it, it sounds like nonsense. It is only possible to accept sex as a part of humanity, instead of trying to remove it, because it does not fit in an all powerful moral theory.
II Defense
It seems to me, that Kant's logic is just incompatible with sex due to its nature. There is something in Kant's theory that seems instinctively true. From thinking about it, I realized the need to make the distinction, that it does not apply to sex itself, but it can be applied to relationships between people. So, it does have its uses when judging if relationships are moral and explaining why many relationships fail. Sex in itself is there for outside of morality, but what matters is that it causes people to do other immoral things and quite often. If two people decide to have sex and have no conflicting commitments, it has nothing to do with morals. But if there are previous commitments (or duties if you wish) or new ones being formed, then applying Kant's logic can give quite justified rules.
There are at least two possibilities, how relationships can be immoral through application Kant's moral theory:
1) It is immoral to pretend to be in a romantic relationship, just to gain easy access to someones body for the purpose of having sex. Because it's using someone as a tool and lying. Both inexcusable in Kant's ethics. Looking at it outside Kant's realm, it also generates false hopes in the other person A and will in no doubt cause them great mental harm, when the sexual appetite has been satisfied and the person B moves on.
2) Secondly, sexual desire is a necessity for a relationship, but it is not enough to form a lasting relationship. This might seem a rather subjective claim, but it has fairly convincing arguments for it. If sexual desire disappears, the core of the relationship is gone and there is nothing else to fill the void. That usually happens with aging. (I once got advice from a fairly intelligent old gentleman – if you marry, marry someone that likes to talk, it's all you'll be doing when your old). If it happens before people are still sexually active, in most cases it is so, then usually the result is having sex with other people. Though it is possible to resolve it by negotiating with the spouse, it usually is not and even more frequently is done without knowledge of the other. Even though the contract of marriage is an arguable one, it is de facto standard of relationships and without agreeing otherwise it is the standard rule set that couples operate in, even if they haven't signed a formal contract. And having sex with others besides the spouse without them knowing is lying in great magnitude otherwise known as betrayal. It is not so because of the “holy institution of marriage”, but because of the expectations of the other. Not to mention, it clearly shows that sexual satisfaction is more important than the spouse. I am not saying this cannot happen if the marriage was based on other things, nor am I saying that unhappy marriage should be sustained, but that successful partnership needs far more than just sex.
Half of the marriages get divorced and before marriage it is quite common to have several attempts to find something called love. Just because it is a failure, doesn't mean its immoral. It just means that love is a lot harder to find than sexual attraction.
Kant fails to prove that sex can be moral because of the institution of marriage. Sex in itself is not immoral and strictly practicing Kant's logic would doom mankind to extinction. However it can still be implemented on human relations in questions regarding sex. Whether in lying to receive sex or mistaking sex as the whole relationship, Kant still has some things to tell us.
