Unlike what is being suggested in the OP's photo (probably from the US based on the spelling on the placards), my understanding is that Australia equalised all government benefits for same-sex couples some years ago, making it the same as for married and de facto heterosexual couples, so there is no practical reason for legalising same-sex marriage (i.e. it won't help them get anything tangible they can't get already). I'm sure someone will correct me if that's not the case, though I'm pretty sure it is because the former government made a point of publicising it.
I don't know why that is not enough, as it is for unmarried male-female couples, and why there is such a strong push for this change. I'm especially weary of the celebrity types and trendy campaigners forcing it down our throats and trying to make it fashionable to have a so-called 'enlightened' attitude. Interestingly, the 'marriage equality' lobbyists seem to shy away from the idea of a plebiscite on the issue and instead try and bully or shame our politicians into just passing a new law by quoting some insignificant poll.
Marriage is an inherently heterosexual institution, not just because it is the current law, and I see no reason to change it. I have nothing against same-sex couples but just believe they do not have a moral right to pervert an age-old institution to suit themselves. Recognising same-sex marriage also weakens the case against recognising polygamy, under-age marriage and perhaps other unions that exist in some parts of the world and may not sit well within our society.
But then, I am a fuddy-duddy at heart who will probably end up on the wrong side of history in this debate eventually. I reckon I'd be in the minority in this forum for sure! I won't lose any sleep over it though.