The answer to this question is inextricably linked to another, equally complicated question: what should be the relationship between church and public life? My own answer is that the church should actively inject Christian values into public life, but should not project Christian expectations. Values such as compassion, mercy, justice, truth and integrity can do nothing to harm society; they are invariably positive for society, and for the gospel. Injecting these values into society through involvement in public life - specifically in the political process - is one means by which the church can be a light to the world. These are our values, both because they are good and because Jesus commanded them, and we should not hide them. We should inject them into all that we can: not hiding the lamp that is Jesus' church, but putting it on a lamp-stand.
Truth is one of our values, and we must always preach the truth about the gospel and God's commands. It is clear from Paul's writings that homosexual lust and-or romance is inappropriate for a Christian. We must not hide or soften that.
But we must be wary of projecting Christian expectations alongside our values. By this I mean the expectations we have of fellow Christians. Homosexual romance is outside God's plan for humanity, and so it is a form of disobedience, of the rebellion which is endemic in all humans. The church should expect Christians to turn away from their sins. But we should not expect or require society to do the same before they have put their faith in Jesus. It is important to consider what spiritual effect actually results from opposing same sex marriage in the political process.
It is more godly to abstain from homosexual romance. But the question of godliness means nothing for a life if it does not follow on from faith in Jesus. It is particularly hollow when godly behaviour, or avoidance of ungodly behaviour, is required by law rather than caused by deliberate choice. Without Jesus, an individual has neither the motivation nor the ability to live for God rather than in rebellion against God. Requiring that they act as if they were living for God will have no effect on their inward attitude, and likely negligible effect on their behaviour. The church should be a light to the world, but it should not set the world on fire to provide better illumination.
I should also raise the point that if the church wishes to legislate against ungodly behaviour, opposing same sex marriage is only the beginning of its pushing for legislation. The majority of heterosexual romance will also have to be banned, as well as most divorces and almost every aspect of the Australian economy. Perhaps this does not seem unappealing. But I find it relevant that Paul instructs the Corinthian church to resist the social norms of ungodly behaviour and speak against them, but not to stamp out those norms. Sin is a part of living in this world. Earthly laws are for maintaining order, not for building up each individual's faith and love for God. And I would argue that projecting Christian expectations into legislation is in fact a subtle form of rebellion against appointed rulers and authorities: rather than trying to build the kingdom of God within petty mortal kingdoms and democracies, the church sometimes tries to create the kingdom of God top-down through mortal legislation. This will never work.
The principal task of the church is to spread the gospel. A significant part of this is, in gentleness and love, making people aware of sin; of their rebellion against God. Ideally they then put their faith in Jesus and live for him, which necessarily involves some changes in their lives. But when the church tries to skip to the end, to push non-Christians towards Christian living (in this case by opposing same sex marriage) it is confusing at best and appears bigoted at worst.
I do not think the church is bigoted. Most Christians who oppose same sex marriage do so because homosexual romance is a form of rebellion against God. But it is impossible for people who do not trust in Jesus to understand that motive, or to differentiate it from hatred and prejudice. This is particularly the case for LGBT (lesbian, gay, bisexual and transsexual) couples and individuals, who continue to experience significant discrimination in Australia. Non-Christian LGBT individuals do not understand where the church is coming from and so they quite reasonably associate the church with the general stream of hatred. So the church, as it tries to project Christian expectations into society, ends up associated with hate rather than love.
This is a real issue for spreading the gospel. It presents the worst possible view of Jesus' church to not only the LGBT community, but to all of society - particularly youth. Young Australians react badly to what they see as blatantly unfair laws. No defence can be made for the church's position without first delivering the gospel, which can never be done effectively in a political context. I believe the church should not oppose same sex marriage. It should continue to make it clear that it does not believe homosexual romance is appropriate. But it must do this in the context of the gospel, not in the context of divisive politics. This will make the difference between the church and society clear, and allow a non-Christian hearing the gospel to subsequently consider same sex marriage in light of the gospel.
The church can and should help care for society through involvement in public life. But it cannot reduce or prevent sin through such involvement; dealing with sin is God's domain. And the church must do nothing to inhibit the proclamation of Jesus' glorious work on the cross to wipe away human sin.