Our opponents are always quick to tell us that they have never against same-sex couples. They are just very concerned about what will happen to the "institution of marriage" if same-sex couples are allowed to marry.
Take the issue of adultery. No one asked for adultery to be left out of the Government's same sex marriage bill. They just decided it was too complicated an issue to deal with and decided to leave it out. It was no grand conspiracy on behalf of evil LGBT activists. It was a silly decision made by bureaucrats that will lead to there being two kinds of marriage (same-sex marriage and marriage).
But in the eyes of our opponents this is all part of a not-so-secret plan to undermine all marriages. Take a look at this mischievous piece on Anglican Mainstream. Here it implies that same-sex marriages will often be open (based on a few vocal US marriage equality activists own marriages and evidence of non-married same-sex couples, of course) and this will seep over into opposite-sex unions leading to the end of civilisation (or something like that).
Of course this all fails to mention one rather important point... opposite-sex couples will still have adultery as a reason for divorce. So what they are implying is not that the Government will be changing the law for opposite-sex couples but simply that those couples will copy those evil gays!
Not only is this the whole "gays corrupt good people" meme writ large but it is also reveals a shocking mistrust of opposite-sex married couples by our opponents. They quote articles which basically imply men are evil sexual monsters (yet again showing signs of feminist and conservative thought slowly moving together) who can only be tamed by a combination of marriage to a woman and anti-adultery laws.
Fundamentally they fear marriage will be changed by married people. They have made their version of marriage into an idol and they simply must force every single married couple to bow before their idol rather than allow those couples to create a marriage that works for them. They talk about "human dignity" a lot but their version of "dignity" appears to mean "Do as we tell you, for we know what is good for you".
That's not dignity. It is servitude. A marriage that is monogamous through choice rather than through fear is one that is far more dignified than what our opponents are offering.