Whether a court will finds a law that bar same-gender couples from marrying violates the equal protection clause is largely determined by what standard of review the court subjects the marriage law to.
Laws that discriminate based on race or gender are subject to strict or heightened scrutiny: the State must demonstrate the law advances a compelling interest and the law must be narrowly tailored to advance that interest. If a law discriminates based upon other characteristics, such as wealth, location, pet ownership, home ownership, etc., the State needs only suggest a plausible legitimate interest in the law. This is know as "rational basis review.
If laws restricting same sex marriage are subject to strict scrutiny, they will be found unconstitutional because any plausible reason for restricting marriage to different sex couples is either not legitimate (e.g., religious reasons, dislike of homosexuals, personal morality), or not narrowly tailored (e.g., encouraging procreation).
If the marriage law is subject to rational basis review, it will likely be found constitutional, at least in California. California has an interest in distinguishing between "marriage" and "civil unions" for the purpose of dealing with federal marriage law, which does not provide legal recognition of same sex marriages.
Thus, the question is (1) is barring same sex marriage gender discrimination; and (2) if not, should there be heightened scrutiny for laws that discriminate against homosexuals (because like race and gender, it is a [relatively, generally] immutable trait that has a history of being the subject of irrational stereotypes, hatred, and malicious legal discrimination.)
Addressing the first question, the majority reasons,
The [current marriage] laws treat men and women exactly the same, in that neither group is permitted to marry a person of the same gender. We fail to see how a law that merely mentions gender can be labeled 'discriminatory' when it does not disadvantage either group
The Court of Appeal makes two incorrect statements: (1) that men and women are treated the same; (2) that "discrimination" means "disadvantaged." In addition, the majority makes a legal mistake: it attempts to determine whether the law disadvantages a gender before determining the correct level of review. The entire point of the heighten scrutiny applied to laws that discriminate based on gender is to take a more careful look to see whether the is, in fact, impermissibly discriminatory.
Men and Women are not treated the same under the current law. A man may marry a woman and may not marry another man. A woman may marry a man and may not marry a woman. One can marry a person with a vagina, the other cannot. One can marry a person with a penis, the other cannot. This is an important difference. Clearly, women and men are being treated differently. If the law suddenly changed to "you can only marry a person of the same gender," it might make a few people upset. Why? because they have a strong preference about which set of sex organs they favor.
The court tries to get around this very obvious discrimination in treatment by phasing the discrimination in the reflexive. They say, each sex is equally unable to marry the "same gender;" however, the term "same gender" only has meaning with reference to the individual's gender. "Same gender" is not a particular category in itself; it requires a subject gender.
Lets try a silly illustration of what should be a rather obvious logical mistake: the legislature passes a law that people are only allowed to purchase and eat plant sex organs that are the opposite of the sex organs they have. Women would be unable to eat fruit, while Men could. Clearly, no one would say each gender is being treated equally in such a situation because women could eat plant stamens.
Ironically, the majority goes on to write, "Rather than dealing in semantics, a court's primary concern in analyzing gender classifications under the equal protection clause is to ensure equal treatment for men and women." It is the court that is using a semantic trick to say the genders are not being treated "differently." Clearly they are: A man cannot marry Tad Benton because of my gender. A woman can. This is a difference!
And this difference is real. Men and women are not completely fungible: "[T]he two sexes are not fungible; a community made up exclusively of one [sex] is different from a community composed of both." (United States v. Virginia (1996) 518 U.S. 515, edits in original.) And as noted above, individuals do not consider men and women fungible in terms of potential for intimate relationships. We care a great deal about the sex of our sex partners.
In response, one can argue that while men and women have some important general differences on average, for the most part, an individual's gender preference is largely immutable. Thus, a restriction of marriage based on gender is only directed at the small percentage of people sexually attracted to their own gender. This is what most of the debate over marriage is about: will we discriminate against homosexuals?
The majority accepts that the current law does discriminate against homosexuals. But finds that homosexuals -- similar to, for example, optometrists -- are not entitled to heightened scrutiny of laws the discriminate against them.
The majority reasons,
For a statutory classification to be considered "suspect" for equal protection purposes, generally three requirements must be met. The defining characteristic must (1) be based upon "an immutable trait"; (2) "bear no relation to [a person's] ability to perform or contribute to society"; and (3) be associated with a "stigma of inferiority and second class citizenship," manifested by the group's history of legal and social disabilities. [Citation.] While the latter two requirements would seem to be readily satisfied in the case of gays and lesbians, the first is more controversial. . . . whether sexual orientation is immutable presents a factual question. The trial court did not conduct an evidentiary hearing . . . .
This analysis is at odds the the majority's position above: that each gender is being treated equally. Either sexual orientation is largely immutable (and, therefore, heightened scrutiny should apply), or it is largely mutable (in which case, the inability to marry a person of the same gender is a real disability based on gender, and not just for self-professed homosexuals).
The Supreme Court's decisions regarding gender discrimination have disregarded legal justifications based upon "fixed notions concerning the roles and abilities of males and females." (United States v. Virginia (1996) 518 U.S. 515.) Beyond the argument over legal classifications and levels of review, the debate over same sex marriage is about the role of men and women in society and how our relationships will be recognized by the government. Laws restricting marriage to opposite sex couples are passed with the intent of enforcing traditional notions of gender roles. They are passed out of fear that society is no longer respecting traditional gender roles. Under the principals articulated by the Supreme Court -- that the State is no longer able to enshrine gender roles into legislation -- laws limiting marriage based on gender are unconstitutional.
Will the California Supreme Court adopt the shoddy reasoning of the Court of Appeal? We shall see.