Against this background of fundamental moral disagreement the liberal idea of state neutrality may have some appeal. But it is neither practical, nor consistent with non-neutralist views of the state. For advocates of natural law, traditional liberal and common good theories of political society, morality does and should constrain the public sphere in so far as policies bear upon basic rights and interests. The state exists in part to promote the  good of society as a whole, and more fundamentally to protect its members' interests from harm or injury arising from the actions of others. On this at least social conservatives and radicals are likely to agree.

How then to proceed? On the one hand, discrimination in law on the basis of private, consensual sexual practice is hard to justify and impossible to implement. On the other hand, society has a right to expect its commonly shared interests to be protected, and these include the norm of two-person, non-incestuous, heterosexual marriage, particularly as that bears upon the needs and formation of children. Reasoning about what policies it is rational for an individual or a government to pursue has to be related to the question of what burdens and harms arise from the effort to encourage or to enforce any given option. Here it may be  useful to make the distinction between value-promoting and value-protecting policies. Natural-law based legislation will seek to protect the good of   heterosexual union open to procreation and it will not promote forms of union other than this. Equally, however, where there is strong demand for alternatives it will consider the cost of opposing this, and where that seems too great in its impact upon civil order and the common good it may elect to tolerate what it cannot endorse.

As Nick Clegg observed, however, and as I have agreed, issues of marriage and the family are subject to social change and it is hard to say where matters currently stand empirically and psychologically, and harder still to predict how they may change in the next few decades. Accordingly politicians and political thinkers need to look hard at sociological as well as anthropological data and resist the temptation to resolve matters on a wholly a priori basis. To acknowledge that, however, is not to say, as Clegg's words might be taken to suggest, that there is nothing to be thought or said beyond observing change — and welcoming or regretting it.

The aim of politics is the promotion and protection of certain social goods, and an emphasis on the rights and liberties of citizens risks overlooking the welfare and interests of the community, including those of its fledgling members, children. Notice that even in caricaturing the 1950s model of marriage and the family, Nick Clegg speaks of the "bread-winning dad" and the "homemaking mother". Perhaps this is an unintended compliment to the virtues involved in co-operatively orienting one's life to the interests of others. Certainly it stands in contrast to a contemporary image of adults asserting their right to have marriage redefined to accommodate themselves without regard to the natural facts of life and the natural needs of children. Which then seems the more caring and generous picture and which the more conducive to the good of society?