"The evidence shows that the movement of marriage away from a gendered institution and toward an institution free from state-mandated gender roles reflects an evolution in the understanding of gender rather than a change in marriage. The exclusion [of gays from marriage] exists as an artifact of a time when the genders were seen as having distinct roles in society and in marriage. That time has passed."So if you live in a country like Saudi Arabia you could legitimately argue against gay marriage since there men and women have totally different defined and unchanging roles. But we live in a free and open society, not bound by rigid stereotyping of gender so it's impossible to find gay marriage wrong. In other words, since the roles of men and women in our society are NOT based on gender: stay-at-home Dads, working Moms, there is no way to base "marriage" on gender.
He also based it on the Constitutional argument that a majority cannot subjugate a minority just based on numbers. The judge said that you can't base laws on majority opposition unless that opposition was based on FACTS. The framers were wise to realize that just because "everyone knows" something, it doesn't mean it's true. Since the lawyers arguing for Prop 8 never proved that gay marriage was harmful based on scientific research, their views were only opinions and in the court of law only facts based on solid research count.
The entire ruling is really worth reading.