The main argument that I encounter is the use of the term "marriage" being applied to a same-sex couple. So... change the terminology across the board.
My support of some form of civil union for gays is based on one case: a couple that had been together for years. We'll call them Bruce and Joe.
They had a home together, lots of belongings, and a great relationship: the ideal that most couples strive for. Then Bruce fell ill with a terminal disease. Joe spent as much time as he could at the hospital with Bruce (like any spouse would). Finally, Bruce succumbed to his illness...
While Joe was at the hospital, grieving for the loss of his partner, Bruce's family made their move. You see, they had never approved of the relationship, and now saw their chance. They broke into the house, changed all the locks, and basically left Joe with nothing but the clothes on his back.
Because Joe and Bruce weren't "married", Joe really had no legal leg to stand on: what Bruce's family had done was perfectly legal, they being the legal "next of kin" and all.
Stop and think about that: the ONLY thing these guys had "done to society" was being in a non-standard relationship. And this guy was completely destroyed, emotionally and financially.
So... a mixed-sex couple can: go to the courthouse, fill out a simple form, present some ID, and pay $80 (what it cost for Spoon and I). Now they're protected from this kinda stuff. A same-sex couple must: go to the courthouse, get multiple forms, probably hire a lawyer, and spend a fair amount of cash for the same protection.
And some of you want to tell me that this is, somehow, "equal under the law"?
Wow. Just... wow.