Progress at home?

Recently, the Supreme Court of California overturned a ban on gay marriage in its state. What's interesting is that California already has laws that give domestic partners all the same rights as married, heterosexual couples. To many religious and social conservative groups, this appears to be another attack on the institution of marriage. It is a fact that social change facilitates and can be facilitated by laws; often it is the case that the society demands a change, and the laws change to reflect and protect these changes. The civil rights movement of the 1970's was a prime example of this.
However, unlike the civil rights movement, the current bid for social acceptance and protection of the homosexual population seeks not only to correct a social wrong, but it also clashes with fundamental cornerstones of our tradition; namely, our definition of marriage.
So the question gets more complicated. Does our legal system have the right to define marriage? To what extent can political or legal decisions be allowed to facilitate social change?
Whatever the answer, it is clear that the upcoming election and storm of political readjustments will have a big impact on gay rights in America, as the people revisit this issue.