I don't oppose amending the tax code to extend joint-filing benefits to both marriages and civil unions. I don't think anyone would have a problem with that. However, I doubt the U.S. Supreme Court would be invoked just to resolve a simple proposed change to the U.S. tax code. I'm pretty sure tax status isn't what's driving the Prop 8 / No Prop 8 debate.
Again, I think this is about whether the state will make a statement to explicitly sanction same-sex couples on par with traditional married couples. It's an equal-status, equal-esteem debate.
I honestly don't know that much about the issues, but I believe there are two separate cases before the Supreme Court. One involves California's Prop 8 - but it has nothing to do with the tax code. The other case has something to do with contesting the Defense of Marriage Act (DOMA) that was signed into law about 20 years ago, and does specifically impact the US Federal tax code as it defines marriage as between a "man and a woman" and thus denies certain benefits to couples of the same sex (ie, they're not considered a "couple" in terms of estate taxes, and they cannot file a joint federal tax return)