Walker, in his decision, writes that "Proposition 8 fails to advance any rational basis in singling out gays and lesbians for denial of a marriage license."This is the key element in the decision. There is no rational basis for discriminating against gays/lesbians who want to marry. The judge amplifies that argument with findings of fact, some of which are listed in the linked article.
Although I am in favor of gay marriage, I also oppose judges creating new constitutional rights by fiat. But that's not what this judge did. Instead, he did exactly what judges are supposed to do. He reviewed a law, and found it to be unconstitutional discrimination. And in my opinion, he is correct. The prohibition of gay marriage is based on long-standing dislike of homosexuality and prejudice against gays -- often founded on religious teachings. If the state is going to be involved in marriages, there's no rational basis for prohibiting two people of the same sex to enter into what is a legal, contractual relationship. In my view, laws specifically banning gay marriage are clearly discriminatory and unequal treatment under the law.
For those complaining that the will of the majority is being ignored, that's why the U.S. was set up as a democratically-representative constitutional republic, with a balance of power between three branches, rather than a pure representative democracy. Sometimes minorities, even small minorities such as homosexuals, need protection against having their rights trampled by the will of the majority.
On the other side, I have long thought that gay marriage rights should come through legislation rather than the court system. Despite the valid reasoning of this judge, many will not accept it, particularly since it goes against such an entrenched tradition, as well as many religious beliefs. I would prefer that homosexuals take a longer view, attempt to change minds, and build a majority consensus in favor of gay marriage rights. I understand why they don't want to wait, but at the same time I think the judicial path stirs up animosity toward gays. It will be interesting to see what happens when this case hits the Supreme Court.