The day we cut marriage adrift from the rock of nature, from the mammalian order of male-female-young, is the day we lose any fundamental reason to deny “marriage equality” to any consenting adults, whether polyamorous* or incestuous. And that way madness lies.

Daniel Brennan, former chairman of the Bar Association in Britain, wrote in March: “After all, if you can abolish the most important precondition of marriage, namely that it requires a person of each sex, why should you be able to retain other preconditions, such as limiting it to only two people? In The Netherlands, where same-sex marriage was introduced in 2001, ‘cohabitation agreements’ have been used to give three-way relationships a measure of legal recognition.”

Nearer home, James Dominguez of Bisexual Alliance Victoria prefers four-way relationships. He told The Australian (21/5/12): “Some time in the distant future we should look at the idea of plural marriage”. In the immediate present, he lives in a bisexual polyamorous foursome, awaiting true “marriage equality” for groups of loving adults.

Gay activist Rodney Croome chooses to close his eyes to both Dominguez and the Dutch, writing in these pages (28/5): “In none of the foreign countries that have allowed same-sex couples to marry has there been a slippery slide to multiple-partner marriage” and “There is no organised push for multiple-partner marriage, including from those people who are in polyamorous relationships”. On the contrary, the logic of homosexual marriage will work its slippery way through our culture, expanding into any permutation of consenting adults. Because if “equal love” trumps gender, it also trumps number.

Another taboo will fall before the inexorable logic of “equal love”. In April, an incestuous relationship came to the European Court of Human Rights. Patrick Stuebing from Leipzig argued that he and his sister had the right to a “family life”.

The case had inspired calls to legalise familial sexual relations. The ECHR refused, saying it was necessary for “the protection of marriage and the family” to punish incestuous relationships. But how long will courts uphold this quaint notion of “the protection of marriage and the family” once “homosexual marriage” has breached the levee of sexual taboo that alone protects the natural order of marriage and family?

In the second half of this year, federal parliament will probably step back from the brink of homosexual marriage. The risk then is that the desire to compromise will make MPs sympathetic to the alternative proposal of homosexual “civil partnerships”. But if it is wrong to take a sledgehammer to marriage, surely it is still wrong to deface it with a chisel? The pseudo-marriage of a gay civil partnership still chips away at the meaning of the thing it mimics and, as we observe overseas, prepares the public mind for full homosexual marriage.

Civil partnerships are part of the death of a thousand legislative cuts inflicted on marriage; others include giving marriage’s core “right to found a family” to single women on IVF or same-sex men using surrogacy. Such counterfeits usurp either the form or the substance of “marriage and family” and must be reversed.

For example, in Queensland under Labor’s Civil Partnership Act 2011, two men can sign up at the Marriage Registry, issue the “bans” and organise a state-licensed celebrant for a public ceremony complete with rings and confetti and the March from Lohengrin. Then under Labor’s Surrogacy Act 2010 the two men can create a baby of their very own, and the Registrar of Births Deaths and Marriages will officially falsify the birth certificate to record the names of the two males as “parents”, with no mention of any mother.

Against this parody, serious politicians must assert one unifying policy: that no law shall be enacted or allowed to stand which tends in any way to diminish the ideal of “a mother and a father for every child”. Such a policy will exclude the artificial creation of babies by single people or same-sex couples and exclude any same-sex institution that mimics marriage.

Is that policy consistent with civil respect for same-sex couples? Frank Brennan, a former chairman of the National Human Rights Consultation Committee, writes: “I think we can ensure non-discrimination against same-sex couples while at the same time maintaining a commitment to children of future generations being born of and being reared by a father and a mother.”

Non-discrimination against same-sex couples is exactly what federal parliament achieved in 2008, when more than 80 pieces of legislation were amended by a bipartisan majority. There is now no unjust discrimination against same-sex couples.

Commitment to children of future generations requires that we discriminate, justly, between two quite distinct social projects: the widespread public task of marriage-and-family, and the rare private commitment of gay partnerships. The number of same-sex couples in Australia is indeed very small, involving only 0.4 per cent of all adults, according to the ABS Australian Social Trends 2009. By contrast, 52 per cent of adults live in a registered marriage.

The 0.4 per cent figure accounts for one third of the 1.2 per cent of Australian adults who identify as homosexual; this compares with the 97.5 per cent who identify as heterosexual, according to the major 2003 study Sex in Australia. Respect even for tiny minorities is necessary, but appropriate discrimination in favour of marriage and therefore in favour of a child’s right to be reared by a mother and a father is also necessary.

* “Polyamory” involves having more than one sexual relationship at the same time, with the consent of all partners involved.

David van Gend is president of the Australian Marriage Forum www.australianmarriage.org This article first appeared in The Australian newspaper (30/5/12) and is reprinted by permission of the author.