Ralph makes a good point.
SF Mayor Gavin Newsome's historic effort to grant gays marriage rights in San Francisco in 2004, while admirable from a human rights point of view, was a disaster for the Democrats nationwide. It was probably a key factor in Kerry's defeat as well as the overall poor showing of Dems in congressional races that year. Which is not to say that Kerry was the best candidate we could have nominated, and he did a lot to ensure his own defeat as well.
In the Prop 8 campaign, the proponents of the ban ran a lot of an ad that featured footage of a somewhat tipsy Newsome (he's since admitted he has a drinking problem) proclaiming "Like it or not, it's gonna happen!" which probably helped Prop 8's passage. Newsome is probably a good mayor in other respects but he should probably clean up his own act before he tries to go national again. He wisely kept a low profile in 2008 and Obama should thank him for that ;-)
As I understand it, California's domestic partner law is nearly the same as the rights offered by marriage. The catch is when federal agencies become involved. For example, Social Security won't give survivor benefits to a registered domestic partner regardless of how tight that relationship was defined by state law. It only recognizes married couples. Same for the IRS, etc... It's an area of legal quicksand. The feds don't recognize domestic partnerships and the states by and large are unwilling to grant gays the right to marry. A compromise solution might be to have all federal agencies grant the same rights to domestic partners as it does to married couples. And perhaps that's what Obama has planned, or will settle upon as an interim solution.
Baby steps, baby.