The particular tack the court took here is interesting. Their ruling that Prop 8 is unconstitutional relies on the fact that, in California, gay couples can get all other effects of marriage except the word by other means -- they are allowed to adopt children, they can share insurance benefits, all that good stuff -- so Prop 8 literally does nothing other than make a semantic distinction between heterosexual and homosexual couples. (Proposition 8 worked a singular and limited change to the California Constitution: it stripped same-sex couples of the right to have their committed relationships recognized by the State with the designation of 'marriage,' . . . while leaving in place all of their other rights and responsibilities as partners -- rights and responsibilities that are identical to those of married spouses and form an integral part of the marriage relationship.) Obviously, says the court, there is no compelling state interest in making a useless semantic distinction like that.
The decision can't be directly applied to a state in which a ban on gay marriage is accompanied by a ban on related civil rights, because there could, theoretically, be a state interest in limiting those rights -- this decision didn't consider that, because the particular case is not rationally related . . . to either of these purported interests [increasing the likelihood of children being raised by two biological parents and decreasing the likelihood of unintended out-of-wedlock pregnancies], whether or not the interests would be legitimate under other circumstances. Proposition 8 could not have reasonably been enacted to promote childrearing by biological parents, to encourage responsible reproduction, to proceed with caution in social change, to protect religious liberty, or to control the education of schoolchildren, since it doesn't actually affect any of those things. We therefore need not, and do not, decide whether any of these purported rationales for the law would be "legitimate," or would suffice to justify Propsition 8 if the amendment actually served to further them.
I get this sense from some of the undertones that the court is rather unhappy with the proponents of the amendment for wasting their time bringing up all this completely irrelevant stuff. We in no way mean to suggest that Proposition 8 would be consitutional if only it had gone further -- for example, by also repealing same-sex couples' equal parental rights, but because this particular amendment didn't affect any other rights, the constitutionality of affecting those other rights simply wasn't considered in this decision.
There's also a distinction between laws that remove access to a right, privilege, or program from groups that had it and laws that create a new right, privilege, or program limited to certain groups. That is, withdrawing a right or benefit from one group but not others is more constitutionally suspect than creating a new right or benefit limited to the other groups. The kind of change in the law is relevant, even if the final results are identical. That means that the fact that marriage was previously allowed to all in California puts Proposition 8 in a less tenable position than a similar ban in a state that already explicitly disallowed certain marriages by other means.
By using their initiative power to target a minority group and withdraw a right that it possessed, without a legitimate reason for doing so, the People of California violated the Equal Protection Clause. We hold Proposition 8 to be unconsitutional on this ground.
(Bold text from ruling as reported by the LA Times.)