American gay rights activist and radio host Michaelangelo Signorile recently wrote triumphantly of what lies around the corner for a country that just re-elected its “First Gay President” -- as a Newsweek cover last year dubbed Obama. Claiming an early victory for his movement in the Huffington Post Signorile proclaimed that “[n]o longer will politicians -- or anyone -- be able to credibly claim to be supportive of gays, and to love and honor their supposed gay friends and family, while still being opposed to basic and fundamental rights like marriage”. Like many other same-sex marriage advocates, Signorile believes the re-election of Barack Obama is a harbinger of nationwide same-sex marriage legislation.
Few can deny that the movement is on the march. At the time of the election four states moved in favour of same-sex marriage and other states are quickly gathering momentum on the issue. Disturbingly, the thought embedded in Signorile’s spiel is that even polite disagreement and opposition to the claims of same-sex marriage advocates amounts to wholesale bigotry and intolerance. Heterosexual marriage supporters vehemently deny this, but many people now think that principled opposition to same-sex marriage is a delusion.
One reason why supporters of traditional marriage are losing elections is that they are losing the war for intellectual credibility. They are too often mired in facile arguments about “tradition”, Bible passages, and bleeding heart litanies about children. Supporters of gay marriage have succeeded in ridiculing these fumbling attempts at a rationale as ignorant and homophobic.
So a robust intellectual defence of the traditional view of marriage by a group of authors which includes Princeton law professor Robert P George is a welcome addition to the debate. Labelled by the New York Times as “America’s most influential conservative Christian thinker”, George, a convert from the ranks of the Democrats, is responsible for the interdenominational manifesto The Manhattan Declarationsigned by a number of Christian leaders in support of traditional marriage, sanctity of life and religious liberty. Sherif Girgis is a Ph.D. student in philosophy at Princeton and a J.D. candidate at Yale Law School. Ryan T. Anderson, who is William E. Simon Fellow at the Heritage Foundation and editor ofthe Witherspoon Institute's journal Public Discourse, is a Phi Beta Kappa and magna cum laude graduate of Princeton University and a Ph.D. candidate in political philosophy at the University of Notre Dame. Altogether a formidable team. 
In 2010 they published a controversial article in the Harvard Journal of Law and Public Policy, putting forward a philosophical defence for the conjugal view of marriage. It gained many responses from all sides of the debate and has proved so successful that they have expanded it into a book to further flesh out their claims and answer their opponents.  The authors start by contrasting, with precision, the “conjugal” and the “revisionist” views of marriage. The conjugal view presupposes an exclusive union between a man and a woman that is bodily and mental and distinguishes itself by its comprehensiveness, while, revisionists claim that marriage is just two (no more, for the moment) people committing to romantically love and care for each other for as long as they see fit.
Girgis and his co-authors steer clear of discussing the moral implications of homosexuality itself to avoid giving critics a pretext for accusing them of denigrating same-sex-attracted people, or making assumptions about their feelings and motives. Instead they argue that the revisionist view (which informs both homosexual and some heterosexual relationships alike) lacks sexual complementarity and comprehensiveness and is therefore vulnerable to internal contradictions and ambiguities.

“[Revisionists] will not see [marriage] as essentially comprehensive, or thus (among other things) as ordered to procreation and family life—but as essentially an emotional union. For reasons to be explained, they will therefore tend not to understand or respect the objective norms of permanence or sexual exclusivity that shape it. Nor, in the end, will they see why the terms of marriage should not depend altogether on the will of the parties, be they two or ten in number, as the terms of friendships and contracts do.”
What is original in their argument is their explanation of why conjugal sex is a worthy human ideal. Arguably, homosexual relationships have many of the same contours that heterosexual relationships do. The authors agree but emphasise that it’s the differences that matter. Sex between a man and a woman differs fundamentally to sex between members of the same sex.
The authors contend that

“[i]n coitus, and there alone, a man and a woman’s bodies participate by virtue of their sexual complementarity in a coordination that has the biological purpose of reproduction—a function that neither can perform alone. Their coordinated action is, biologically, the first step (the behavioral part) of the reproductive process. By engaging in it, they are united, and do not merely touch, much as one’s heart, lungs, and other organs are united: by coordinating toward a biological good of the whole that they form together.”
With this as an anchor, they tackle other common arguments which are used to subvert conventional marriage. They contend, for example, that understanding marriage as a merely emotional union threatens marital norms such as permanence and exclusivity:

“But why should these be limited to two people? Indeed, how could they be, if we form emotional connections with various loved ones- parents, siblings, close friends – and by various activities? Romantic emotional unions do have a different quality from others and are clearly important for marriage, but emotional hues are hardly enough to mark the difference in kind between marriage and ordinary forms of friendship.”
To the common objection that, if marriage is about making babies, then infertile heterosexual marriages are not true marriages, the authors respond that conjugal acts have a meaning in themselves, not just because they produce babies.

“Here the whole is the couple; the single biological good, their reproduction. But bodily coordination is possible even when its end is not realized; so for a couple, bodily union occurs in coitus even when conception does not. It is the coordination toward a single end that makes the union; achieving the end would deepen the union but is not necessary for it.”
They illustrate this thought in typical American fashion by way of a baseball analogy:

“Infertile couples and winless baseball teams both meet the basic requirements for participating in the practice (conjugal union; practicing and playing the game) and retain their basic orientation to the fulfilment of that practice (bearing and rearing children; winning games), even if that fulfilment is never reached.”
The authors comment on the fallacious argument that equates traditional marriage laws to laws banning interracial marriage, and point out that the current debate is not about who gets to marry but what marriage is really about.

“First, opponents of interracial marriage did not deny that marriage (understood as a union consummated by conjugal acts) was possible between blacks and whites any more than segregationists argued that some feature of the whites-only water fountains made it impossible for blacks to drink from them… By contrast, the current debate is precisely over whether the kind of union with marriage’s essential features can exist between two men or two women.”
Supporters of traditional marriage tend to be flummoxed by the challenge, “How would same-sex marriage affect you and your marriage?” But Girgis and his co-authors show that same-sex marriage would have very real effects. First of all, real marriage would be obscured by altering its civil definition:

“People forming what the state called ‘marriage’ would increasingly be forming bonds that merely resembled the real thing in certain ways, as a contractual relationship might resemble a friendship. The revisionist view would distort their priorities, actions, and motivations, to the harm of true marriage.”
Religious freedom will be at risk. The authors point to the alarming state of religious freedom in places where a same-sex marriage culture takes precedence and provide a number of worrying examples. They point to Massachusetts where Catholic Charities was forced to give up its adoption services rather than place children with same-sex couples, and to Damian Goddard, a Canadian sportscaster who was fired from his job for supporting conjugal marriage on Twitter. They provide many other examples.
In positing “comprehensiveness” in union as the crucial element in marriage, the authors distinguish themselves from classic defences of heterosexual marriage. Classical natural law theorists like Aristotle and St Thomas Aquinas considered homosexual acts as “perverted” uses of human sexual organs. As an adherent of the philosophical “new natural law” school of ethics, Girgis and co-authors view this argument as “fallacious”. They don’t argue, like the classical natural law theorists, that certain parts of the human body have in-built purposes for which they are designed but instead simply argue that if a relationship isn’t comprehensive overall then it isn’t a marriage. The authors neatly avoid the dispute between the two views, presumably to devote more space to the question of same-sex marriage.
Notwithstanding this shortcoming, the authors traverse a solid path between the libertarian view which argues that government should have no role in marriage, and a liberalism that wants government approval and benefits for emotional bonds. Against both positions, they argue that marriage consists of rights between parties that ought to be protected by governments because “wherever reasonably possible, parents are entitled to bring up their own children-and children have a right to their own two parents’ care, as affirmed by the United Nations Convention on the Rights of the Child”.
Though their argument is mainly philosophical the authors also provide helpful social science research to bolster their case. Taking studies from both left and right leaning think tanks they conclude that children raised in intact families headed by two biological parents fare best in areas of educational achievement, emotional health, familial and sexual development, and child and adult behaviour. They also conclude that lower-income earners are the hardest hit by a failing marriage culture. What is Marriage? offers a sober and resounding answer to that question. Marriage, as it has been understood for centuries, is a unique and comprehensive institution that joins mothers and fathers to their children and to each other. The authors show that it is a rationally defensible cornerstone of Western civilisation and a bedrock of societal stability. In confronting the challenge currently facing marriage in parliaments, courtrooms and schools around the world Girgis, Anderson and George put forth a clear and formidable case for one of our most sacred traditions.  Shawn Murphy recently completed a journalism course at Monash University, in Melbourne.  Corrections: This article has been corrected to reflect the role of Sherif Girgis as lead author of What is Marriage? and the spelling of his name.