While I understand and appreciate the argument that is put forward by some about the nature of marriage in Australia having its roots in religious tradition, there is no need for this to be so. Particularly since we are told that we live in a secular society, and that there should be separation between Church and State. A lot of people get married by civil celebrants, who are (as I understand it) expressly forbidden to offer religious services.
Given that the State has moved to remove any arbitrary discrimination against people in same sex relations vis a vis things like superannuation and inheritance issues, I don't see any particular need for the State per se to continue to define marriage in religious terms.
They could do this by simply allowing the rules and regulations to apply to two persons rather than people of opposite=gender.
The religious difficulty could be removed by removing the right of religious ministers to also be marriage celebrants. All marriages could be performed by a registrar of the state, and should a couple wish it, a religious wedding could take place later...but not have any legal force.
This would allow same-sex weddings in a secular sense, but not require religious groups to compromise their principles.
While I think a case can be made for Christian same-sex marriage I am also realistic in that I can't see a conservative church like my own (Anglican) church agreeing to it in my life time.
But that is no reason to prevent the wider non-religious community from availing itself of such a provision