This article does a good job explaining it's hardly that trivial. It reminded me of Steve Yegge's blog post about legalizing marijuana[0], and a simple concept becomes impossibly complicated when you consider all the practical scenarios and edge cases. Hopefully, you'll never be in a situation where the government has to rule on something like a disputed inheritance, but it has and will do so, often basing its decision on previously established precedent.
What struck me was the logical step that this is a good defense against the slippery slope arguments, because gay marriage is ultimately an incremental step. We have laws and legal precedent for marriage between two people, and we're effectively just saying those laws can apply to a marriage of two people of the same gender. A typical Republican counterargument to the legalization of gay marriage is that you open the door for marriage to be between multiple partners, or between children, or animals, or whatever absurdity. It's impossible for gay marriage to trivially lead to say, marriage to an animal, because the state will have to define and rule on an entire cohort of new edge cases. If you're gay and married and have a child and you die, it's pretty uncontroversial that your partner would maintain custody of the child. But if you are married to your dog and adopt a child and die, does your dog get custody of the child? That's basically absurd.
So, I feel this essay gives me a good answer the next time a right-winger asks me if the government allows gay marriage, what's to stop them from allowing other sorts of bizarre marriage scenarios: the government would have to come up with a bunch of different rules that are way more complex than gay marriage, and we probably won't want to do that.
[0] http://steve-yegge.blogspot.com/2009/04/have-you-ever-legali...