Quite simply, the judges got it wrong.
In exercising all sorts of legal jujitsu to conclude that there is a fundamental, constitutional right to homosexual marriage, the majority's 127 page opinion can be summarized as follows:
Existing constitutional right to marriage + Social acceptance of homosexual relationships = Constitutional right to homosexual marriage.
The problem with this is that new rights shouldn't be discovered in the Constitution by judges just because society accepts certain arrangements at certain points in time, unless society's approval has been expressed through the legislative or voter initiative process. In contrast, the California legislature enacted a liberal domestic partnership law but purposely stopped short of calling such arrangements "marriage." Similarly, in 2000, the voters of California (although a minority of Californians) approved a ballot initiative defining marriage as being between a man and a woman. Right or wrong, the legislative and voter initiative processes in California have both rebuffed expanding the definition of marriage to include homosexual unions.
In fact, the controlling law in California states that fundamental, constitutional rights are those "which are, objectively, deeply rooted in this [society's] history and tradition [and] implicit in the concept of ordered liberty such that neither liberty not justice could exist if it were sacrificed." (Washington v. Glucksburg (1997) 521 U.S. 702, 720-721, as cited approvingly in various California cases).
There are no grounds to assert that a right to homosexual marriage has been deeply rooted in our society's history and traditions.
This is established much more eloquently in Justice Baxter's dissenting opinion in the In re Marriage cases, which can be read here, starting on page 128:
In re Marriage
Unfortunately, the majority's imprimatur on this decision has mainstreamed the idea that the gay marriage debate is about constitutional rights (although even the plaintiff's briefs were too afraid to go as far as asking for a new right, the majority went ahead and did this for them). Once this happened, the No on 8 campaign obtained a sort of faux-moral high ground by gaining the ability to label traditional marriage advocates as haters who want to take away fundamental rights. This is all symptomatic of the fact that we haven't developed a good way talk about what rights we as a society deem to be fundamental as a separate issue from whether rights are being taken away or not.
Uninformed No on 8 partisans will misunderstand what I am saying here, so the following paragraph is for them:
It may very well be that the time has come to expand the definition of marriage to include homosexual unions. But, if that is to happen, it needs to happen by the people of California expressing their opinion on this subject through the legislative process. To date, the people of California have actually rejected the idea. Circumventing this process by having the courts suddenly find that your cause has actually been a fundamental right all along does lasting damage to the constitution and democratic process. And claiming that the newly invented right to homosexual marriage is actually an ancient and fundamental right, and then using this as a brush to paint all Prop 8 supporters as haters and bigots, is beyond the pale.
If you don't believe me, Barack Obama pretty much said the same thing in his now famous 2001 radio interview where he explained where civil rights advocates went wrong in relying on the litigation process to broaden the definition of rights.