Things, I am afraid, are looking rather grim.

From where I sit, the case for legal recognition same-sex marriage has landed blow after unanswered blow against the traditional alternative. The campaign has appropriated for itself the powerful contemporary language of rights and equality. There have been television ads with celebrities; and articles by top journalists such as David Marr. The Greens seem determined to pursue this issue and to lever their parliamentary influence to this end.

The genius of the campaign has been the way in which it has made its case seem simple and unanswerable. It has made its appeal to middle Australia – that great, largely inert mass of people who don’t have time to think about the issue but have a keen sense of fair play. And it sounds like quite a reasonable ask: all gay and lesbian couples are asking for is the same recognition by the government that heterosexual couples receive.

What’s more, the campaign has cast those who might uphold the traditional view as bigots driven by religious zealotry, determined to impose their irrational and medieval views on the rest of the community come what may. I am fully expecting this article to lead to such accusations.

Small wonder there has been little by way of response. We in the churches have been, I think, afraid to make the public case in defence of traditional marriage. All we have offered is a stunned silence.

Partly this is because the way in which the rules of the debate have been framed by the emotive language of the campaign for change. Arguments which appeal to religious traditions and texts are given no place whatsoever. And since Christians believe what they believe about marriage on the basis of Scripture, it seems that we cannot say anything germane to the public issue.

I understand and sympathise with those who argue that actually it would be better if the churches withdrew from advocacy for legal recognition of traditional marriage. After all, we don’t believe that we are dependent on the state for the reality of marriage. We are too reliant on the state to protect these things. Perhaps we are better letting the government do what they like, and just modelling in our own communities a different kind of relationship, which we will call ‘marriage’. And perhaps too our concern over this issue distorts our witness to the community about the gospel of Jesus. It makes us too easy to categorise as people lacking grace and compassion, whatever the reality.

But marriage is not merely something that we know about from the Christian revelation. It emerges from our very human nature. And it is a divine gift to all humankind, not just to the Christian community. If we are interested in the wellbeing of the Australian community, I would suggest, we cannot sit idly by and watch the institution of marriage disintegrate.

So: it is time for those of us who would support traditional marriage to work hard at this issue. We must do much better than we have with recent public debates (I am thinking of SRE especially) in speaking to the general community with non-defensiveness, intelligence and compassion. It will certainly take courage, because of the censoriousness of the opposition.

There is an opportunity, however, because in their assumption that there is nothing that can be said against them the advocates of the revisionist campaign have majored on rhetoric and emotion and neglected to put forward a plausible case. So far, the case seems to be: ‘we want what you have. That is, we want equality with heterosexuals as far as the legal recognition of our relationships goes. Most of all we want to be able to use the word ‘marriage’.’

What no-one seems to notice is that the proposed revision of marriage laws involves … a revision of marriage. That is, they wish to change the meaning of marriage in order to have what we now call marriage. Only, if the law grants to them ‘marriage’, it won’t be the same at all. It will have become something essentially different.

As it is currently understood, marriage is not merely the expression of a love people have for each other. It is (in the words of scholars Girgis, George and Anderson of Princeton and Notre Dame Universities) “a comprehensive union of two sexually complementary persons who seal (consummate or complete) their relationship by the generative act—by the kind of activity that is by its nature fulfilled by the conception of a child.”

This is not a random definition; nor is it one based in divine revelation (though it accords with the teachings of many religious traditions). It is the meaning of marriage that emerges from almost all human civilisations across history; and which reflects who human beings are in their very bodily selves. There is a union which only persons of complementary sex can share. Only this two can become ‘one flesh’ – and that is not some spooky, mystical phrase: it is a matter of tangible reality.

Were same-sex relationships to be admitted as ‘marriages’, this essence of marriage itself would have to be held to be something other than what it is. This is what some pro-revision advocates themselves think. Andrew Sullivan, a leading academic advocate for same-sex marriage, writes that as far as he is concerned, marriage has become“primarily a way in which twoadults affirm their emotional commitment to one another.” Brandeis University’s E.J Graff thinks that recognition of same-sex unions would change marriage so that it would “ever after stand for sexual choice, forcutting the link between sex and diapers.”

Here’s the thing. The advocates of same-sex marriage are counting for success on that great staple of Australian politics - the apathy of the great majority of Australians. It sounds to most of us as if recognizing same-sex marriage won’t affect or harm most of us at all. It sounds as if denying gay and lesbian couples this ‘right’ is petty and discriminatory.

But that doesn’t reckon with the fact that the way in which marriage is described will be completely changed. It will be something else. The distinctive orientation of marriage towards the bearing and nurture of children is to be dissolved. In its stead, we have a view of marriage which places sexual choice and emotional commitment at the centre. Under this definition (which is rarely articulated), there is of course no reason why marriage rights should not be granted to polyamorous relationships, or indeed any other type of sexual relationship. Indeed, it is unclear even why sexual activity should be the focal point – why couldn’t long term housemates or inseparable golfing partners likewise seek recognition at law for their relationships?

What is missing from the revisionist case is a clear and reasonable definition of marriage as they would like to see it – one that is deeper than just ‘choice’ or ‘emotions’. This is because for the most part advocates want the wider community to think that the change will be minimal in impact.

It won’t be.  The definition of marriage is changed, that will affect all of us. It will further destabilise the bedrock of our social order, as the liberalisation of divorce laws has tended to do – to the measurable and visible detriment of many of our fellow citizens.

Marriage is a public, not a private matter – which, by advocating so strongly for change, revisionists themselves tacitly acknowledge. It is not simply therefore a matter of allowing a freedom for others. It is a matter of determining what best promotes the flourishing of Australia’s citizens.