Selwyn Duke at Orthodoxy Today:

... these leftists cannot say what marriage is, how can they be so sure about what it isn’t? If they cannot offer a definition they’re certain is right, how can they be so confident that the right definition is wrong?

But the point is this: the court obviously doesn’t accept the definition of marriage embraced by most people worldwide today. If it did, it would have ruled as indicated earlier. Yet there also is no noted alternative definition by which to go. Thus, it seems that before the judges could rule on the right to this thing called marriage, they’d have to rule on what this thing is in the first place. So have they ruled that there is a right to they-know-not-what.

... This is why the left’s actions do, in fact, threaten marriage. To fail to respect the institution’s time-honored definition and also refuse to offer any alternative definition is to seek to destroy the edifice without a plan for what will take its place. It is to imply that marriage can mean anything. And if something can mean anything, it means nothing.

As for conservatives, they have been suckered again. Without even realizing it, they have allowed the left to frame the debate — as a matter of rights — when it is first and foremost a matter of definitions. To argue it as a matter of rights is to lose the debate; to control the definitions can render that debate irrelevant.