Why do we need marriage? I think it was Robin Williams who summarized it as, Why shouldn't gay couples be as unhappy as everyone else?
Your mileage may vary, but: I needed marriage because I might need divorce.
Like a lot of same-sex couples with kids, we decided that it would be best for our family to have a stay-at-home parent for early childhood. The complications for us included finding health insurance for our family, as well as selecting one of us to leave the work force and lose her connections and skills. The 'mommy track' isn't just for two-mom families, but the risk to an unmarried parent in making that sacrifice is larger.
If I had needed the practical benefits of divorce, which include making the outcome fair for the parent who loses out on current and future income to take care of the family, this would have been vital to my future. Marriage protected me, while my kid was small, from the possibility that I would be thrown out of my home by my girlfriend, who had found someone she liked better. As long as we're 'girlfriends', I'd have no way to recover financially.
For the 55,000 kids being raised by their same-sex parents, marriage is a big deal.
Not only because they have as much right as all the other kids to see their families reflected in the curriculum of public schools.
Not only because tax systems favor married couples, allowing them to pass more wealth to their kids.
And not only because California's same-sex couples in every race and ethnic group are raising their kids on less money than their married peers.
Because our kids didn't choose their parents, and because we're not perfect people, it's only fair to open up marriage to all couples who want it.