It doesn't. What it threatens is the idea that heterosexual marriage is a unique institution. Those who are opposed to it believe that by allowing the institution to be redefined to include a broader scope, it loses some of its meaning.. . The analogy that might help those in favor is private property. Your back yard is special because it's just for you and those you invite. If everyone had access to it, then it loses the exclusivity that makes it special.. . Personally, I think that's the wrong analogy, because it's really like saying: if everyone else is allowed to have their own back yard, then my back yard stops being special. And that's just another way of saying: I'm better than they are, and they don't deserve what I have. Which is basically how most forms of prejudice survive -- out of fear that somehow we're going to lose if someone else wins.. . It's the same problem that most of the world/species has in most areas of thought. By believing the world is a zero-sum game, some people become convinced that if someone else wins, they have to lose. But the world is broader than that, and there is nothing stopping everyone from being able to find happiness -- except fear and intolerance. So, the question becomes, is that a trait inherent to humans, or is there hope for the species?