The term “natural law” often emerges within the inescapable debates over same-sex marriage, where it is just as often criticised, rejected, and even ridiculed.  Natural law is important to many critics and opponents of same-sex marriage, yet it is poorly understood and often misrepresented in the public debate, in part because the natural law tradition – once the mainstay of Western ethical debate – is now deeply foreign to our contemporary way of thinking.  Yet quite apart from issues like same-sex marriage, the natural law theory or set of theories are viewed by many as a neglected treasure of the Western ethical tradition with insights that circumvent a number of the challenges and pitfalls of other ethical systems.
Natural law theory is an approach to ethics that focuses not solely on consequences of actions or on abstract absolute rules, but on the study of human nature and the practical investigation of which actions, pursuits, and ends are most appropriate to the elusive and mysterious goal of “happiness”.
While natural law is historically associated most closely with the Stoics of Ancient Greece, its formative principles are also derived from the work of Aristotle, and – from the collective influence of the Platonist, Stoic, and Peripatetic schools – were taken up and further developed and elaborated by Christian philosophers, most notably of the Scholastic method. 
Academic ethics eventually diverged from the natural law perspective, but the theory has undergone something of a resurgence in recent decades, with new interpretations and approaches emphasising natural law as a viable alternative to popular utilitarian systems.  Like utilitarianism, natural law has developed a number of interpretations and theories, albeit with common underlying themes. What is natural law? The word ‘nature’ has been abused and misconstrued to the point where most of us define it immediately as:

“the phenomena of the physical world collectively, including plants, animals, the landscape, and other features and products of the earth, as opposed to humans or human creations.”

But in this case we are interested in another definition of nature as:

“the basic or inherent features, character, or qualities of something.”

If we observe, for example, that nearly every human in our acquaintance has two arms and two legs, we might be inclined to conclude that this is an inherent or essential quality of being human.
But when we meet someone who has less than two arms and two legs, does this not prove that “two arms, two legs” is not an inherent quality of being human at all? Doesn’t it imply that human beings can have any number of arms and legs, or even none at all?
This example is, after all, reminiscent of a famous anecdote in philosophy of science: not so long ago there was a common expression “as white as a swan”. In 82 AD the Roman satirist Juvenal described something as rara avis in terris nigroque simillima cygno "a rare bird in the lands, and very like a black swan"; that is to say, non-existent.
In the Northern hemisphere swans are indeed white. But on their arrival in Australia, European explorers discovered the existence of black swans. Thus, the phrase “as white as a swan” disappeared from common use.
What is the difference between a black swan and a person who is missing a limb? From a certain philosophical perspective there is no difference – both indicate that we cannot make presumptions about the nature of things; in fact it would be better not to talk about “nature” at all, not to impose value judgments on things that do not meet our narrow expectations.
Natural law does not accept such a conclusion, and neither, for that matter, does modern science.  In the classification and ordering of species, science is more than happy to make distinctions between the black swan and the person missing a limb, determining that the former is a distinct species, the latter an abnormality within a species.
On the same basis, natural law would utilise observation and reason to arrive at the tentative conclusion that having more or less than two arms and two legs is not inherent to human nature.  This suspicion could be borne out by further examination – finding that many individuals missing a limb have arrived at this state through accident, illness, or injury.
But what of those who are born without a limb? Here our observational powers reach a limit until more precise and intricate investigation can reveal pre-natal injury, illness, or genetic abnormality.  Even without knowing the cause, an observer would still likely conclude on the basis of rarity and non-heredity that the absence of limbs is not an inherent quality of human nature. What about same-sex attraction? How does this apply to same-sex attraction? Would natural law categorise same-sex attraction as an inherent quality of being human, as an anomaly, or – like the black swan – as indicative of a distinct species or sub-species in its own right?
While it may be feasible to reach a conclusion on the basis of non-heredity and rarity, the fact is that natural law does not approach attraction or desire from quite the same perspective as something like the loss of a limb. Rather, the whole point of natural law theory as an ethical system is to guide and inform those who are not content to accept their own desires at face-value, but who wish to shape their desires according to a more complete understanding of what it is to be human, with the goal of what Aristotle enigmatically terms eudaimonia – a term not entirely captured in the translation “happiness”, but which is often rendered “flourishing”, and in a literal sense implies the protection of a benevolent spirit.
Happiness is an inferior translation because we know from our own experience that we can be extremely happy by achieving a desired end, regardless of how good for us that end may be, or how satisfying we may find it with the benefit of hindsight.
Rather, eudaimonia denotes a state of well-being at which we may arrive quite apart from our subjective sense of happiness at any given moment.  “Happiness” might mean sitting watching TV every night with a good wine and some delicious food; but eudaimonia might require instead some physical and mental exertion, self-improvement, or the difficult sacrifices involved in caring for loved ones, building relationships with friends and family.
Satisfying our desires can make us feel better, but eudaimonia is about becoming a better person; where the direction of that betterment is not widely understood or well instantiated in our society.
This is why, on the one hand, I do not regard natural law theory as an immediate solution or answer to the cultural and political challenges raised by issues such as same-sex marriage or the gay rights movement. Eudaimonia is not something that can be legislated or imposed by government decree. Nor is it something to which we all subscribe with appropriate diligence; as both advocates and opponents of same-sex marriage have pointed out: the dismantling of marriage as a social institution began long before anyone ever thought the words “same-sex marriage”.
On the other hand, natural law theory is as difficult and unstinting as one might expect of any system aimed at profound self-cultivation.  It asserts on the basis of observation, experience, and reason that procreation is one of the goods one must respect if not achieve for the sake of eudaimonia. Most parents will attest that the responsibility and challenge of raising your children changes you as a person. It does not make you “happier” because it is deeper than merely satisfying a desire or set of desires.
At the same time, natural law is as multifaceted as human nature itself, and no single factor is exaggerated to the exclusion or harm of others. Add monogamy and fidelity to procreation, but limit procreation out of respect for human life, and already a host of ancient and modern practices are repudiated: from divorce and concubinage, to IVF and surrogacy.
All of this is anathema to those who find their desires in conflict with “outdated” laws and mores and their obscure natural law bearings; a conflict to which we all are subject to varying degrees.  But what are our desires, ultimately? What is their foundation? From where do they obtain their urgency and validity? And once we discover that our own desires are subject to internal examination, interrogation, and cultivation or neglect, on what basis do we then proceed to form them?
If we arrive at a point where the law prohibits nothing but the most egregious or offensive behaviours, in some sense still nothing will have changed. We will all still face the most basic question of how to live, how to order ourselves and our desires.
Many people make the mistake of thinking that natural law is simply an appeal to “what happens in nature”. According to this misinterpretation, the varied proponents of natural law theory have erred in reaching conclusions on the basis of insufficient data, an error that can be remedied through more systematic surveys of contemporary human sexual behaviour.  If modern behavioural sciences indicate that diverse sexual behaviours are well-represented in various populations, then we must accept this diversity as part of nature.
If modern psychology now claims that same-sex attraction is a positive variation of human sexuality, then who are we to deny it?
But the conceptualisation of human nature is not derived only from observation of human behaviour; pederasty was, after all, by no means unknown in the ancient Greek context in which natural law theory arose. Nor is the goal of eudaimonia synonymous with the much lower bar of “not mentally ill” reflected in the removal of homosexuality from the Diagnostic and Statistical Manual of Mental Disorders in 1973.
The purpose of natural law ethics is to help us determine which of a diversity of possible desires a person ought to cultivate and pursue for the sake of a higher end; this, in vivid contrast to ethical theories that aim at finding a mutually agreeable balance between the diverse and conflicting desires present within society. 
Proponents of same-sex marriage may well be inclined to say “You can keep your ‘eudaimonia’, I’d rather have political change”, and indeed it is hard not to feel some sympathy for those of us raised in an era of moral agnosticism when presented with the daunting, seemingly callous face of an ethical system that runs directly against the grain of contemporary mores. 
This is, I suspect, the most important lesson to take away from current debates over same-sex marriage: that our society has steadily embraced an ethical approach almost entirely oriented toward the fulfilment of individual preferences and desires, to a point where any system that disputes the validity of our desires is viewed as some kind of nightmarish anachronism like a medieval torture device.  
This ethical shift did not begin with the gay rights movement, and will likely not finish with it.  But while legal arguments and public debates rage, it is perhaps more important than ever to recognise the depth of the ethical divergence, and contrarily, the immediacy and profound challenge of eudaimonia for those ready to pursue it.  Zac Alstin is associate editor of MercatorNet. He also blogs at zacalstin.com