From Peter Bolt (in the comments on the post):
A marriage is simple: a man and a women in a committed sexual union, public, recognised, agreed to, acknowledged.
Biology matters. No matter how many rights we may or may not have as human beings, a man does not have the ‘right’ to say he is a woman. Biology says he is not. (and vice versa)
The strength of a liberal democracy, such as Australia used to be, is that, despite our differences, we are treated equally.
Therefore, in order to maintain equal treatment, we need to ensure that differences are clearly described/defined.
Same-sex unions are not marriages, by definition. If same-sex unions want recognition from Government, fair enough in a liberal democracy. But this recognition should not be from saying there is no difference. But by saying there IS a difference.
The definition of marriage as between a man and a woman, clearly differentiates this from a union between two people of the same gender. This definition of difference must be maintained, so that equality of treatment might be ensured.
Christians believe marriage is between a man and a woman. Biology matters because God created us male and female. As citizens in a liberal democracy we may say it is fine to press for Governmental recognition of same-sex unions, to prevent any disadvantage for our fellow-citizens. But this should not be through saying there is no difference, that all unions are the same. No, true equality of treatment (in a liberal democracy) needs to be on the basis of saying, no, there are genuine differences. Marriage between a man and a woman is never a same-sex union. Both may be, or even should be, recognised at law, but this should not come through changing the definition of marriage. It should come through clarifying the enormous difference, and then ensuring that those who are so vastly different are still treated fairly.
From Mark Baddeley (also in the comments; SSM=Same sex marriage):
I’m not opposed to SSM because I think it is immoral. I think immoral people, criminal people, good people, law abiding people, all should be able to get married. I think thieves, fornicators, murders, paedophiles, bitter people, racists and the like should be able to get married. I think people who aren’t sure if they are going to stay married for life and those who intend to run their marriage as an ‘open marriage’ should be able to get married. I have strong views as to how successful I think some of those marriages are likely to be, but I think there shouldn’t be any legal bars.
I oppose SSM for the same reason why I oppose people marrying their pet, or marrying themselves, or marrying their child (explaining why that one is in that set will take some words, but I can offer them), or marrying the number seven. Or wanting to be ‘married’ to everyone that is their friend on Facebook – it isn’t what marriage really is, and the more that kind of thing is done it weakens our grasp of what the institution is, and so weakens the institution.
For political reasons—that is, because the current government is flailing around without clear purpose, and being dictated to by a minority party—this is a bigger issue than usual in Australia at the moment.