The only argument with an ounce of credibility that I have heard is that to change the definition of 'marriage' is quite a big practical task. Marriage is currently defined as the union between a man and a woman, and always has been, so to go and change all the legal documents, dictionary definitions, legal definitions etc is a lengthy process.
Not that I'm saying for a second that the logistics of it should stop it actually happening. Just that it is kind of a relevant point!