THE Prime Minister believes in marriage. This should be obvious to anyone who has followed his arguments in favour of extending marriage to gay couples. Most people probably agree that marriage is good for society. If this is so, then it is grim news from the Office of National Statistics that only half of those who had babies in 2011 were married. If the trend continues downwards, marriage will soon be a minority choice.

This will harm society: unmarried parents are far less likely to stay together than the married. A recent estimate puts the cost of coping with the fall-out of unmarried parenting and high divorce rates at £40 billion in benefits, and legal and health costs.

Supporting marriage means taking a stand against other arrangements, and no one is willing to risk doing that. It is not easy to advocate marriage. Many politicians and church people are divorced, me included. It feels hypocritical to beat a drum for the institution that we have failed. But perhaps it is worse to collude with the indiscriminate trumpeting of "equality" as the one social virtue by which all others must be measured.

Equality means that no lifestyle is to be preferred over any other. All deserve equal rights, even though not all deliver desirable outcomes. Since the passage of the gay-marriage Bill through the House of Commons, there has been a resurgence of the demand for civil partnerships for heterosexuals - a move that could lead away from the social stability for which David Cameron has argued.

Heterosexual civil partnerships would protect the material interests of the individuals concerned, while not requiring the solemn commitment. This appeals to those for whom even civil marriage is too "religious", because it involves promises and a marriage certificate.

From the "equality" standpoint, it does not matter if the costs of this anarchy cripple the economy even further. It does not matter if the scale of relationship breakdown causes long-term pain and distress, especially to children. Equality comes first. But the virtue of equality is not quite as virtuous as it seems. An equality that invites individuals to choose a path that harms them, their children, and society is surely not a virtue, but self-indulgence.

This is one issue on which the Church should have a clear message for the whole of society. It is a disgrace that the Church has shown so much concern for its own rights in respect of gay marriage that its plea for the victims of the equality agenda has gone unheard.

The Revd Angela Tilby is the Diocesan Canon of Christ Church, Oxford, and the Continuing Ministerial Development Adviser for the diocese of Oxford.