This was a good decision. California's Proposition 8 declaring that marriage is limited to those based on one man and one woman is not Constitutional.
The two main reasons are 1. there is no rational basis for the state to deny marriage licenses to two men or two women, and 2. mere moral disapproval of another person's behavior is not, by itself, justification for declaring that behavior illegal.
CNN also reports this decision: Judge rules California's ban on same-sex marriage is unconstitutional.
From the NY Daily News: Federal judge Vaughn Walker overturns California's Prop 8 in win for same-sex marriage.