It goes without saying that a gay couple who wanted to live their lives together before SSM was legalised did so anyway. In countries where it hasn't been legalised they are doing so right now. So what changes when it does become legalised? Well, they get rights in regard to each other that they didn't have before. And that's about it.
Why on earth would someone attempt to argue against that?
But....goes the cry...marriage is a sacred institution! Well, for a small minority it is. For the last year that figures are available, in Australia 4 out of 5 marriages were conducted by a civil celebrant. So no, it's not regarded as a sacred matter by the vast majority.
Australia - share of religious and secular marriages 2018 | Statista
All we are left with is a petulant cry by some that 'hey, marriage is something which
we define. And
our definition is the one that counts!'