Chait offers a tour de force of logic and comes to the following explanation:
Good ideas don't always defeat bad ideas, but they usually, over time, defeat non-ideas.
That just about sums it up. It's odd to me that the very first argument used against marriage equality in the courts was simply definitional (you can look it up in my anthology). And the very last argument is definitional as well: the lame, focus-group-tested "I believe marriage is for a man and a woman." It is a rational reflection of pure discomfort, an expression of gut-level conservatism, which one should respect for what it is. But as an argument, it isn't much. Which is why in a debate, they lost.