While I was living in Europe a few decades ago, I remember a Belgian friend going home to get married. I recall being very surprised when I learned that he would have two weddings.
One was according to the laws of the state and the other followed the ritual of the Church. The two marriages were separated by several weeks. They were conducted by different celebrants, at separate venues. Each had its own guest list and reception afterwards.
After a while I realised that it was normal to have two weddings in such European countries. And it made sense. One was to satisfy the law of the land and the other was a sacrament of the Church. Two distinct means to achieve separate purposes.
In Australia, we don't properly appreciate this distinction. As a result, we tend to conflate the two. That is despite the fact that the way we conduct marriage is actually not that much different to the Europeans.
What happens for Australian couples opting for a church wedding is that the priest or minister facilitates both the legal and sacramental marriages. He or she is separately licensed by the state and the Church, and the tasks performed for each of these bodies are almost mutually exclusive.
But by conflating the civil law with religious ritual, we create confusion that makes it easy for the Church to claim authority that rightfully belongs to the state. In other words, the Church makes demands regarding sacramental marriage, which of course is OK. But it often weighs in on civil marriage as well, which is different.
Therefore I think it is problematic for religious leaders to be urging a no vote in the Australian Marriage Law Postal Survey. The Survey has nothing to do with their domain of sacramental marriage, and they are being disingenuous if they act as if it does.
If the state weighed in on sacramental marriage and required church celebrants to marry same sex couples, the Church would cry foul. In any event, this would not happen because the Church is protected by existing religious freedom legislation that allows it to discriminate against same sex couples.
If, on the other hand, the Church believes that it can insist on its definition of civil marriage because it has a stake in the 'moral order' of society, the Survey will provide an interesting test for the moral authority it retains in the wake of its conduct regarding the sexual abuse of children it was responsible for.