Wish I could make an essay out of this, but I'm afraid this time it's just too simple. The etymology of 'marriage' never intended to be gender specific, age specific, race specific, creed specific, or even species specific. It doesn't even have to be biological. As a valid alternation, when you mix melted steel into another batch, you are literally marrying the two batches of steel. It's a marriage. =)
But seriously, why should gay marriage bother anyone? They're not out to convert the world or ostracize the recorded and established findings of scientists or anything like that, they're just spouses. No agenda but their own, totally minding their own business. Shouldn't we?