I think this is a bad idea.
So does this guy who wrote a looooong post about it.
And then I wrote a comment under that post, which I've reproduced below, slightly edited with some headings (because I know most of you won't read the actual comment)...
Thanks for writing this Nathan. I attend a Qld Pressie church, so I found it very interesting.
What were acceptable marriages in the New Testament?
I pretty much agree with your conclusions. I think it's instructive to remember that there are no instructions about weddings in the New Testament. Marriages were just accepted as marriages on the basis that they were recognised as marriages by society.
Do Hindus who convert to Christianity have to remarry? (Hint: No.)
This is reflected in our day, where a Hindu couple who were married in a Hindu ceremony are not required to "marry for real" if they become Christians - their original vows stand. Perhaps more explicitly because of their non-Christian teaching around marriage, Mormons who convert to Christianity are not required to "marry for real" either. Of course, this is because marriage is a creation ordinance.
Would a changed Marriage Act still marry people? Would a "Presbyterian marriage" be a real marriage?
That being the case, even if the Marriage Act were to change to allow the legal creation of same-sex marriage, anyone legitimately married under that act would be legitimately married in God's eyes, and should be in the Church's eyes. I don't believe it is faithful to the Bible to create a new category of "Presbyterian marriage" that is not recognised by the state. If it's not recognised by society as a marriage then it's not a marriage. The only way I could see a "Presbyterian marriage" as legitimate would be if Presbyterians were forbidden to marry by the state.
Do we accept every "marriage" society accepts as marriage?
Let me say, though, I am not suggesting that a same-sex couple would be legitimately married in God's eyes, even if society agreed that they were. Even now there are marriages that we as the Church do not consider legitimate. We as the church can determine which is a biblically legitimate marriage and which is not, even if society disagrees. But I don't think we can declare a couple married if it is not recognised by society.