Christians, especially Catholics, who think the law should not restrict marriage to one man and one woman, should be careful about what they wish for.
The Commonwealth Constitution forbids the federal government (but not the states) from prohibiting the exercise of any religion. Political considerations and the legal difficulty arising from the uncertainty of the definition of "religion" make it unlikely that the relevant Section 116 will ever be invoked.
However, there are other means by which freedom of religion can be inhibited, notably, any anti-discrimination legislation. If the present definition in the Marriage Act is extended to include unions of same sex couples, it is conceivable that a church might be forced to provide a marriage service for a same sex couple even though its religious teaching would regard this as a mockery of the Sacrament of Marriage.
Further, if same sex unions are recognised as legal marriages, why not polygamous unions also, and even unions of humans and animals? In this age of constant incremental change, redefining marriage to include same sex couples could be the first step on a very slippery slope.