Contemporary arguments about sexual morality generally assume utilitarianism (or consequentialism) and nominalism. Consequentialism holds that the morality of an act is purely determined by the goodness or badness of its consequences; utilitarianism further specifies that this is to be evaluated in terms of net pleasure/suffering brought to society as a whole. Nominalism says that there are no universals. Universals are just convenient fictions of language allowing us to name groups of similar things, but there is no actual nature or essence shared by things named by the same universal. Thus, arguments about the meaning of "male", "female", and "marriage" have no meaning from which we can derive moral conclusions. There are only individual things which we classify as "male, "female", or "marriage" for convenience.
This combination has been used as a sledgehammer against traditional definitions of marriage and prohibitions against homosexual acts. Arguments that marriage or the conjugal act are different "kinds" of things, with inherent purposes are met with scoffing and reductio ad absurdum arguments: "If marriage is about procreation, why do we let infertile couples marry?"
This is just one instance of the broader movement to demystify sex. Sex has no essence, no nature, no meaning or purpose beyond the subjective meaning and purpose attributed to it by the participants. Sex is not magic. In itself, sex can't be distinguished morally from other kinds of physical activity, and different sex acts cannot have different moral meanings apart from the moral meaning attributed to them by the participants. So, for instance, there can be no inherent difference between the act upon which the continuation of the species depends and "an unnatural act of the Oscar Wilde type". To say otherwise is to be superstitious and bigoted.
The problem is that this proves too much. (And here comes my reductio ad absurdum.)
We're already seeing in some boundary-pushing publications that this approach to sexual morality can't really make a coherent argument against bestiality. Provided that no physical harm is done to the beast, the only arguments to be made would be that it is somehow "unnatural" or that it debases the human participant. But these all rely on bigoted essentialism. If the participant doesn't find it debasing, then it isn't debasing. (Maybe an animal rights advocate would make an argument based on consent, but this just shows that the idea of attributing rights to animals is absurd.)
Similarly with (consensual) incest. Again, arguments that it confuses relationships in a way that is "unnatural" depend on essentialism--taking seriously kinds of relationships and sex as a particular kind of act, regardless of how that act or those relationships are viewed by the participants. People will bring out arguments about the genetic risks of inbreeding, but this can no longer determine the question. For one thing, procreation can be easily avoided, and genetic screening and abortion could be used as a backup. Anyway, arguments about gay sex and parenting have already determined that such risks of general social harm don't count for much against individual desires. For example, the social cost of the AIDS epidemic and its causal connection to male homosexual buggery is about as clear as it gets, but any suggestion that take measures to limit this activity is denounced as bigoted. Arguments based on psychology can be similarly put aside--if there are no essential kinds of relationships and acts, there can be no clear cause and effect in psychology--or at least, nothing clear enough to rule out relationships and acts that are consensual and otherwise appear "healthy".
Ultimately, it is hard for me to see even how liberals can consistently uphold the age of consent and strictures against pedophilia, if they consistently evaluate sexual morals by this withering standard. Society has bunkered these strictures in the language and logic of consent, which makes them less vulnerable to the reductionist logic. Within liberalism, consent is the one remaining absolute when it comes to sex, and kids can't offer meaningful sexual consent to adults. Still though, this only makes sense if you presume that sex is magic--different in kind from other kinds of acts. Children can consent to giving handshakes and high fives and playing board games, can't they? So what sets sex apart? Or, for that matter, what sets sex between teenagers apart from sex between a teenager and an adult? If minors can't give consent to adults, how can they do so to each other? Ultimately, our arguments still rely on the belief that sex is something different in kind from other acts, and the age of consent protects the innocence of minors from being violated, even though, we've otherwise discarded the concepts of "innocence" and "violation". The backup argument would be claims of psychological harm, but, again, is the causal link strong enough to be proven definitively and distinguished from the harm caused by social stigma? Homosexuals and transgendered folks have all kinds of psychological problems, but if you argue that these are related to their sexual proclivities rather than caused by social stigma, you are a bigot.
I'm not arguing in favor of these disgusting acts, nor am I saying that the advocates of the acceptability of gay sex are actually advocating these things. I am only arguing that the rightful disgust we feel at these acts only makes sense within a thicker moral framework for sex than the reductionism that is brought out against traditional arguments against homosexuality. That reductionism is being used selectively and inconsistently. When applied consistently, it cuts at the logical foundations of sexual limits that most liberals still hold to.
Despite what we say when we want to justify certain acts, we still implicitly treat sex as if it were magic and inherently meaningful prior to the intentions of the participants. We still believe in innocence (in some form) and understand that some kinds of sex can violate it. We're only dismissive of those ideas when it suits us for the purpose of justifying our own actions or those of a sympathetic minority.
