What they keep on skating over is that equal marriage isn’t something cooked up by atheists and agnostics. It’s also – as Lynne Featherstone has pointed out – supported officially by Quakers, Liberal Jews and some Unitarian Churches, not to mention many people of other faiths that officially take a different view.
Insisting that the state continues to make illegal the sort of marriages which Quakers, Liberal Jews and some Unitarians want to carry out isn’t defending marriage against the state. It’s about saying that some religions should be specially favoured by the state at the expense of others.
Let’s keep it simple and leave aside the question of what role an Established Church should or shouldn’t have, and just consider this:
Why should the law say what the Catholic Church believes on marriage is legal but what Quakers believe is illegal?
Believing in religious freedom means believing the law should let Quakers do marriage the way they wish just as much as Catholics.
So opponents of equal marriage should be up front and say it: they want the law to discriminate against some religions. They don’t want religions to be free to choose their own practices, nor do they want religions to be equal under the law. They want some religions to be able to enforce their views on others with the force of law.
If that’s what you believe, you should say it. I still think you’d be wrong, but at least you’d be frank about what you want.