I can well understand that gay couples, who surely experience love for their partner in the same way as anyone else, want to be able to declare their love publicly and have their union recognised by society. And frankly, in a country where gay relationships are perfectly legal, and discrimination against gay people is rightly outlawed, I cannot see why the law should not provide a means for them to declare and formalise their union.
On the other hand, I would prefer that the state provided a proper legal form and ceremony to gay unions without actually calling them ‘marriages’. I think I would have agreed with the bishops of England and Wales back in the day when they supported legislation for gay ‘Civil Unions’, though I concede the term is less than elegant. But it seems a rather lazy sort of law-making to simply change the definition of ‘marriage’ to make it include other types of relationship, when what you really want to do is recognise those other relationships in their own right. It would be better to save the name ‘marriage’ for the thing it has long defined: the shared life-project of a man and a woman who commit to a lifelong partnership, intending to have children together and to create a home and family. But the time for that has probably passed. The campaign is now for gay 'marriage', and nothing but marriage.
Many of my colleagues among the bishops are very strongly opposed to redefining marriage in the civil law. There is no doubt that they are right to be concerned. There really is a tutelary function to the law. In other words, if the law says that a gay union is a marriage, many people will cease to see that there is any difference. That has probably already happened in the case of de facto marriage: the law has gone a long way towards treating it as legally equivalent to marriage, and that may well explain in part why so many people no longer see any point in actually marrying. The law does shape attitudes. But that is not the first consideration when thinking about law.
In Catholic tradition the function of civil law is to serve the common good, to maintain as well as possible the peace, security and rights of all the people. Law is not there to impose ideals or to teach a moral code, but to keep society functioning smoothly. Thus, although adultery is a major sin, it has rarely been illegal in Christian countries. Law has to take account of how people actually live. It will permit or regulate certain behaviours when the attempt to suppress them would cause more harm than it remedied. It also has to make special provision for minorities, because it really is better for us all to live in a society that looks after everyone, not just the majority.
In any case, the law around marriage already differs substantially from the Christian understanding of it. I've mentioned that adultery is not illegal. Australian law permits divorce at the behest, almost at the whim, of one party. The state marries people that the church won't, principally the divorced. So if the state should come to permit same sex marriages, it won't be the first time that the church's practice will differ from the law of the land. Not by a long shot.
I'm inclined to agree with views I saw attributed to Archbishop Prowse of Canberra; I would prefer that 'marriage' was not redefined in the law, but if it is, it won't be the end of the world. It may allow the church to make better known its teaching on marriage, by way of a greater contrast with ‘marriage’ as it is in Australian law.
Finally, let’s note that even the notion of ‘a loving union’ that I’ve been talking about does not express all we Christians understand by ‘marriage’. There is no sense there of the sacramentality of marriage, that the couple receives grace from God for living out their life of love and fidelity. There is no element of their making a solemn commitment before their God as well as before their friends. There’s a vagueness about permanence and indissolubility. It’s light on the idea of the couple sharing in the work of the Creator by bearing and raising children. Let’s face it, there is already a lot of daylight between marriage as understood by Catholics and ‘marriage’ in Australian law and majority social consensus. We’ve learnt to live with that.