"Married" in what sense? If they're atheists, they probably didn't opt for a church wedding. So that means they're married for the state's purposes, the very situation we've been discussing here. I repeat, the state does not require any spiritual dimension to endorse a relationship, nor should it. But the resulting union is a "marriage" in the same sense that a contract between me and my mechanic is.
And don't post then retreat. It just makes you look small.
But from the point-of-view of the state, other distinctions can be made. All relevant religious traditions hold a union of man and woman in marriage as sacred. Those strong traditions are very, very valuable to the preservation of liberties. For hundreds of years, our several states have supported an institution of marriage, based on the traditional ones. There still exists a strong cultural tradition, even while there is now a lesser prominence of religion among the populace.
I believe it to be one of the many stupidities of modern liberalism that it is pushing to change the legal and broader cultural institution of marriage. It is beyond stupidity and approaching tyranny to change it by judicial fiat.