In 1776 when the founding fathers of the the United States of America drafted a Declaration of Independence, they inserted a truly revolutionary assertion, ``We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights, that among these are Life, Liberty and the pursuit of Happiness.'' Written at a time when modernism was flowering throughout the western world and the order of old Europe was beginning to melt away, it makes explicit a radically transformative idea: that the individual is the irreducible particle of society. Prior to the modern era, the atomic unit of society was considered to be the family or some extension thereof such as the clan or the tribe or, in a few notable cases, some sort of religious grouping such as the Church or the Umma. But in all cases, society was founded on what was thought to be the smallest unit capable of effectively reproducing society. Unlike oak trees where a a single acorn, given enough time and all requisite material needs, can repopulate a forest, human beings need a male and a female to come together in sexual union to reproduce. For this reason, among others, marriage between a man and woman has held a special place in every ancient and medieval society throughout recorded history. In such societies talk about a same-sex union on par with marriage would have been an abusurdity. A gay couple, incapable of sexual reproduction, is unable to reproduce society. But when, as in modernism, the atomic unit is the individual and reproduction is no longer seen as an essential part of what makes society a society, it follows that what makes the relationship between man and woman called marriage unique is no longer an important consideration. Rather than the reproduction of society, what lies at the core is the an individual's life, the freedom of an individual to do as he or she pleases and the ability of an individual to pursue that which makes him or her happy.
In a world where ``nothing new is under the sun'' the emergence of gay marriage as an innovation is nothing short of astounding. While homosexual acts and relationships have certainly been recorded, and even tolerated, by various societies in both the ancient and medieval eras, there is no historical evidence that any society before the modern era accepted the life long commitment between two people of the same sex as moral, legal or economic equivalent to marriage. Admittedly, ancient Greece and Rome did have something of a tradition of pederastry. After all, Parmenides apparently kept Zeno as his paidakai well into Zeno's middle age. It is also true that various cults used homosexual acts as part of their rituals and in some ancient cultures ritual sodomy of the conquered by the conquerors was widespread. But none of these homosexual relationships or acts of homosexual behavior, whether consensual or not, are ever portrayed in the written record as anything approaching the relationship of marriage.
The largest reason for this, I suggest, is the understanding of most societies that reproduction was both vital to its own continuance and an understanding that this continuance was an unequivocal good. Religious prohibitions against homosexual behavior, regardless of context, occur in the Holy Writ of most religions. In some religions, such as Judaism, these prohibitions occur alongside prohibitions against various forms of birth control such as coitus interruptus. The most obvious inference is that reproduction was seen as an essential part of marriage and, in turn, marriage and family was seen as essential to the survival of society. By the Hellenistic era, this view was given the most clear voice by Aristotle. Just as an acorn is required to develop into an oak tree, a family of husband and wife is necessary to develop into society. Just as an oak tree reaches its fullest potential in a forest, humanity reaches it's fullest potential in society. Society, then, is the final end of what begins in the family.
This view, explicitly or implicitly, has driven most of the western world's understanding of marriage up through the modern era. It wasn't until the dawn of the modern era that the family, or one of its various extensions, was replaced by the individual as the irreducible unit of society. In modernism society no longer exists to continue itself, but rather is created by the agreement of various individuals out of self interest in that society will help the individual survive (Hobbes) and live a more pleasant life (Locke). No longer is society a necessity for the development of the human person. Rather society is contingent choice made by individuals. Once society becomes an option rather than a necessity, the sort of relationship that reproduces becomes an option rather than a necessity; the type of union of between man and woman that ends in reproduction becomes optional.
We see this change not just in regards to the way marriage is viewed with regards to gay people, but also with regards to the way that marriage between members of the opposite sex are viewed. Procreation within marriage has become a question of preference rather than being seen as a essential part of what it means to be married. Marriage itself becomes an option to the adult member of society rather than an expectation for most members of society. Marriages have become easier to dissolve and the expectation that people will remain married long enough to raise children to adulthood has dissolved. No longer having the meaning that it once did, marriage becomes something new, a commitment arrived at entirely for reasons of romantic love, which has as its basis the idea of society as a collection of individuals rather than a unity of human nature. Once one has accepted the modern understanding of society, gay marriage will eventually follow as there is no basis within the modern understanding of society itself by which one can reject the idea of two people of the same sex marrying each other. Any opposition must come from an older (or different) understanding of society, one that does not accept the individual as the atomic unity of society.
In the US, the most widespread view of the family rather than the individual as the atomic unit of society is that offered by Christianity. It should be no surprise to anyone familiar with the history of religion that most forms of Christianity firmly resist the implications of atomic individualism such as gay marriage. Aside from the language in the letter of Saint Paul to the Romans that echos the language of natural law used by Aristotle, Christianity's anthropology is rooted in the idea of the completion of the human person in the unity of male and female as a symbol of the way that Christ and the Church will be united on the last day. If anything is surprising about the relationship between Christianity and gay marriage, it is the increasing number of Christian denominations that have embraced the practice. These Christian sects that have embraced the idea of marriage between two people of the same sex, it would seem, have embraced the idea of human liberty being the highest human good.
But that Christianity, with some exceptions, has an anthropology fundamentally opposed to accepting any relationship between two people of the same sex as fundamentally equivalent to marriage says nothing about the how Christians should view the acceptance of such relationships by the state. Unlike some religions, the most ancient, and most ingrained, political philosophy espoused by Christianity, to render unto God what belongs to God and to Caesar what belongs to Caesar, implies that Church and State are two very separate entities. There is no clear and indisputable imperative within Christianity to form the state into the likeness of the Church. When Christianity first became an official state religion in the late Roman era, it was only because the Roman empire had first become Christian. And even then, it was not until much later that Christian norms began to become the law of the land. Speaking of this dichotomous early Christian political philosophy, Remi Brague wrote, ``the cities of God and the cities of the devil are two sorts of comportment, not two political entities; the city of men is to be guided by moral rules, at times inscribed in the law, but their organization does not derive from a religious law.'' [Brague, Remi. The Law of God: The Philosophical History of an Idea. Trans. Cochrane, Lydia. The University of Chicago Press. 2007. p. 37.] While conflating the `cities of men' with the `cities of the devil' probably errs in being too Augustinian, Brague's central point that Church and State were not only considered to be two separate entities, but were considered to be two separate kinds of entities cannot be overstated.
This analysis gives rise to a question of no small importance. What relevance does the way that the state runs its affairs have to Christianity? Rather than offering this as a rhetorical question with an implied answer of `nothing (or `everything) as various groups would have, I think it important to notice that neither the Christian scriptures nor the oldest traditions of Christianity directly address this question. Rather than being a question with an obvious, unequivocal answer, it is a question fraught with nuance and shades of gray. Further, even if the question is answered with some sort of affirmative, it then leads to the inverse question. What relevance does the opinion of Christianity have to the state in a liberal democracy like the US? While the answer to this question is probably not as clear cut as many secularists would like, it is certainly more clear cut than the former question. As discussed above, regardless of what Christians may think, the moral and political liberalism that underlies the US constitution ends with no sound reason by which gay marriage can be rejected as illicit. To deprive same-sex couples of seeking out marriage would be to deny them the pursuit of happiness on their own terms.