During the course of recent debates in the British Parliament over the proposed legalisation of gay marriage, it has gradually become apparent that the proposal itself is impossible. For legislators have recognised that it would be intolerable to define gay marriage in terms equivalent to "consummation," or to permit "adultery" as legitimate ground for gay divorce.

Thus, despite the telling squeamishness in much contemporary conversation on homosexuality, which invariably steers away from its physical aspects, the legislators have been forced tacitly to admit the different nature of both gay sexuality and of gay sociality. But such an admission destroys the assumption behind the legislation and the coherence of what the legislation proposes to enact.

The assumption behind the legislation is that "fairness" must involve the application of universal rights to each individual in the same way and in the same respects. But this admission reveals that, in the current instance, such application would prove grossly unfair, inappropriate and unrealistic.

The coherence of the legislation depends upon erasing the clear distinction between civil union (which is already available for both straight and gay couples) on the one hand, and marriage on the other. But if the binding and loosing of gay and straight marriage are stipulated in different ways, then in effect such a distinction has been reinstated.

The suspicion arises that the proposed legislation before the British Parliament seeks only an empty change in nomenclature - this is borne out by the fact that the intended circumscription of gay marriage is so diluted as to render it indistinguishable from gay civil partnership.

Why, then, should Christians worry, if this is all just a matter of terminology? Can we not live with differing definitions of marriage? Perhaps, in order to safeguard the churches from pressures to conform to the norm, we should now welcome a withdrawal from the churches of their rights as a civil marriage broker. This would leave the churches free, in their turn, to claim that only natural and sacramental marriage are genuinely "marriage," while state marriage is mere civil union. They could trump secularisation by declaring that the era of civil marriage had been a failed experiment.

This may, indeed, be the direction that the churches now need to take. However, the graver fear surrounding the new legislation is that secular thought will not so readily let go of the demand for absolutely equal rights based on identical definitions. In that case, we face an altogether more drastic prospect. Not only would "marriage" have been redefined so as to include gay marriage, it would inevitably be redefined even for heterosexual people in homosexual terms. Thus "consummation" and "adultery" would cease to be seen as having any relevance to the binding and loosing of straight unions.

Many may welcome such a development as yet a further removal of state intrusion into our private lives, but that would be to fail to consider all the implications. In the first place, it would end public recognition of the importance of marriage as a union of sexual difference. But the joining together and harmonisation of the asymmetrical perspectives of the two sexes are crucial both to kinship relations over time and to social peace. Where the reality of sexual difference is denied, then it gets reinvented in perverse ways - just as the over-sexualisation of women and the confinement of men to a marginalised machismo.

Secondly, it would end the public legal recognition of a social reality defined in terms of the natural link between sex and procreation. In direct consequence, the natural children of heterosexual couples would then be only legally their children if the state decided that they might be legally "adopted" by them.

And this, I argue, reveals what is really at issue here. There was no demand for "gay marriage" and this has nothing to do with gay rights. Instead, it is a strategic move in the modern state's drive to assume direct control over the reproduction of the population, bypassing our interpersonal encounters. This is not about natural justice, but the desire on the part of biopolitical tyranny to destroy marriage and the family as the most fundamental mediating social institution.

Heterosexual exchange and reproduction has always been the very "grammar" of social relating as such. The abandonment of this grammar would thus imply a society no longer primarily constituted by extended kinship, but rather by state control and merely monetary exchange and reproduction.

For the individual, the experience of a natural-cultural unity is most fundamentally felt in the sense that her natural birth is from an interpersonal (and so "cultural") act of loving encounter - even if this be but a one-night stand. This provides a sense that one's very biological roots are suffused with an interpersonal narrative. Again, to lose this "grammar" would be to compromise our deepest sense of humanity, and risk a further handing over of power to market and state tyrannies supported by myths both of pure human nature and technocratic artifice.

It is for this reason that practices of surrogate motherhood and sperm-donation (as distinct from the artificial assistance of a personal sexual union) should be rejected. For the biopolitical rupture which they invite is revealed by the irresolvable impasse to which they give rise. Increasingly, children resulting from anonymous artificial insemination are rightly demanding to know who their natural parents are, for they know that, in part, we indeed are our biology. But this request is in principle intolerable for donors who gave their sperm or wombs on the understanding that this was an anonymous donation for public benefit.

The recipe for psychological confusion, family division and social conflict involved here is all too evident and cannot be averted. In this instance we have sleep-walked into the legalisation of practices whose logic and implications have never been seriously debated.

From this it follows that we should not re-define birth as essentially artificial and disconnected from the sexual act - which by no means implies that each and every sexual act must be open to the possibility of procreation, only that the link in general should not be severed. The price for this severance is surely the commodification of birth by the market, the quasi-eugenic control of reproduction by the state, and the corruption of the parent-child relation to one of a narcissistic self-projection.

Once the above practices have been rejected, then it follows that a gay relationship cannot qualify as a marriage in terms of its orientation to having children, because the link between an interpersonal and a natural act is entirely crucial to the definition and character of marriage.

The fact that this optimum condition cannot be fulfilled by many valid heterosexual marriages is entirely irrelevant, for they still fulfil through ideal intention this linkage, besides sustaining the union of sexual difference which is the other aspect of marriage's inherently heterosexual character.

John Milbank is Research Professor of Politics, Religion and Ethics at the University of Nottingham, and Director of the Centre of Theology and Philosophy. He is also Chairman of the ResPublica Trust.