What marriage will mean
The Government's proposals fail to define the substance of marriage and thus fail to distinguish rationally between relationships and arrangements which are and are not to be treated as marriage in law. Presumably, it is assumed that marriage will mean an exclusive and permanent sexual companionship between any two adults. But if the proposals are implemented, what will marriage come to mean?
The unspoken rationale for changing the meaning of marriage is most plausibly based on radical choice. Given that civil partnership already provides an equivalent legal framework with similar practical effect to marriage, the campaign for same-sex marriage is best seen as an attempt to confer full social recognition on minority sexual identities. The basis for that recognition lies in the view that no social disadvantages should attach to different sexual choices. Rather, individual decisions in this area should merit equal public respect. The problem is that this rationale extends far beyond exclusive and permanent sexual companionships. It calls into question other definitional boundaries as well.
The exclusivity of marriage will be undermined The proposal to extend marriage to same-sex partners assumes that marriage is exclusive to two people. Why should this not be a matter of personal choice? From a purely pragmatic and political point of view, if a minority defined by sexual orientation can get marriage redefined to suit their wishes, why can't other minorities? Traditional Islam allows a man up to four wives. Polyamorists argue for no limit to the number of partners one may choose. These are real options in the social ordering of human sexuality.
Sexual interdependence expresses the exclusivity of marriage. The fact that the human race is relevantly male and female in this context makes sense of our intuition that sexual exclusivity matters. Between them, a man and a woman represent humanity and its future in a way that two people of the same sex do not. If two people of the same sex can enter into marriage on the grounds of sexual intimacy and companionship, then why not three?
The permanence of marriage will be undermined In spite of the relatively easy availability of divorce, marriage is still lifelong in intention. Permanence serves important personal, social and economic goals. The fragility of marriage is a major cause of harm in twenty-first-century British society. The problem is that stability requires the genie of autonomous choice to be kept firmly inside its bottle. Why should one be so foolish as to sign away one's future choices for the rest of one's life? If one is allowed to choose the terms on which one enters marriage, why should one not choose a less permanent form of relationship?
 On this cultural development, see Anthony Giddens, The Transformation of Intimacy: Sexuality, Love and Eroticism in Modern Societies, Polity Press, 1993.
 Matthew Parris expresses this ideal with customary clarity: 'The day that the battle for homosexual equality is won and over will be the day when a man, straight or gay, can boast that he chose.', The Times, 21 April 2012.
 See, e.g., the arguments of Ronald Dworkin in Sovereign Virtue, Harvard University Press, 2000, ch. 14. Dworkin makes clear that he considers 'prohibitions' on same-sex marriage to breach each person's foundational right to ethical independence: Justice for Hedgehogs, Harvard University Press, 2011, p.369.
 Around fifty, mostly Muslim majority, countries recognise polygamous marriages, either generally or for Muslims under their personal law.
 The growing construction of an 'identity' among the small British community of 'polyamorous people' can be seen at http://www.polyamory.org.uk/. The first goal of the (US) Polyamory Society includes 'to support, defend and promote the social institution of polyamory': http://www.polyamorysociety.org/mission.html (both accessed 30 August 2012).
 Philip L. Kilbride, Plural Marriage for our Times: a reinvented option?, Bergin & Garvey, 1994. See also the manifesto, 'Beyond Same-Sex Marriage' athttp://www.beyondmarriage.org/full_statement.html (accessed on 17 September 2012).
 Jean Hannah Edelstein, 'Why shouldn't three people get married?', The Guardian, 6 September 2012.
 Quite apart from the emotional impact, the cost of family breakdown to the public purse must be phenomenal. The obvious methodological caveats notwithstanding, The Relationships Foundation has recently calculated a figure of £41.7bn a year: Counting the Cost of Family Failure (2010).
Dr. Julian Rivers has contributed regularly to Cambridge Papers for the 20 years of their existence. He has published widely in constitutional law, legal theory, and law and religion studies. He is Professor of Jurisprudence at the University of Bristol Law School, an editor-in-chief of the Oxford Journal of Law and Religion, and a member of the advisory board of the Ecclesiastical Law Journal.