What makes gay marriage different from straight marriage, fundamentally speaking?

I was reading Pinker's discussions on government in "The Blank Slate", and I had an epiphany on the psychological motives for marriage. Let me know if you feel I'm wrong please, but it's the right to property and inheritance. The comfort in life with knowing that your stuff is protected even after death, that people can't kill you off just to take your stuff. That you've got someone to back you up in the case you stop living. I think there's a minor paradox with marriage with no kids because what happens when both partners die? But I think that has got be one of our instinctual needs: the need to inherit or own our property even after death. Do you think this is off base?