A key concern for opponents of redefining marriage is that it would redefine parenthood, and change social attitudes to mothering and fathering.

This week, the National Post reported on a case in Canada (where same-sex marriage has been legalised since 2005) where a court has upheld a decision to declare a gay man the legal father of a girl, even though he is not her biological father, and it was his former partner who actually inseminated the girl’s mother.

The man, known as Mr. H to protect the girl’s identity, raised her with his partner Mr. R for the first three years of her life, and it was only a discriminatory part of Alberta’s family law that denied him — and any other non-biological father who is not married to the mother — the legal status of “parentage based on intent,” a common feature of adoption, surrogacy, and other kinds of non-biological parenting.

It also imposes responsibilities. Nicholas Bala, a family and children’s law expert at Queen’s University Faculty of Law, said the decision reflects and reinforces two legal trends: one to give greater recognition to “social or psychological” parents and the other to pin down the baffling new realities of reproductive technology, in which the old categories of mother and father no longer always fit.

There have been a series of similar cases in Canada, where it appears the ideal of a mother and a father for every child has been completely erased from the law.

There are many concerns surrounding male same-sex parenting. Surrogacy and deliberately denying a child access to its mother are problematic in all sorts of ways.

But leaving these concerns aside, redefining marriage to accommodate same-sex couples (rightly or wrongly) inevitably changes social ideas about parenting and the role of mothers and fathers, as the law would make mothers and fathers optional for children raised in married households.

Now, there are obviously some people who believe gender and biology are utterly irrelevant in parenting, and have no issues with changing the law in this regard.

But let’s face it: this is a very significant change, which goes against the collective human wisdom of almost all cultures across thousands of years, with potentially severe consequences for a large number of children, and so we all have a duty to think long and hard before starting down this path.

Blaise Joseph is a third-year commerce student at the University of New South Wales with a strong interest in social policy. Blaise is originally from Canberra, the centre of politics and the public...