From the Legal Opinion (pdf)
…the legal issue we must resolve is not whether it would be constitutionally permissible under the California Constitution for the state to limit marriage only to opposite-sex couples while denying same-sex couples any opportunity to enter into an official relationship with all or virtually all of the same substantive attributes, but rather whether our state Constitution prohibits the state from establishing a statutory scheme in which both opposite-sex and same-sex couples are granted the right to enter into an officially recognized family relationship that affords all of the significant legal rights and obligations traditionally associated under state law with the institution of marriage, but under which the union of an opposite-sex couple is officially designated a “marriage” whereas the union of a same-sex couple is officially designated a “domestic partnership.” The question we must address is whether, under these circumstances, the failure to designate the official relationship of same-sex couples as marriage violates the California Constitution.
It also is important to understand at the outset that our task in this proceeding is … only to determine whether the difference in the official names of the relationships violates the California Constitution…the constitutional validity of the current legislative provisions.
Although, as an historical matter, civil marriage and the rights associated with it traditionally have been afforded only to opposite-sex couples, this court’s landmark decision 60 years ago in Perez v. Sharp (1948) 32 Cal.2d 7114 — which found that California’s statutory provisions prohibiting interracial marriages were inconsistent with the fundamental constitutional right to marry, notwithstanding the circumstance that statutory prohibitions on interracial marriage had existed since the founding of the state — makes clear that history alone is not invariably an appropriate guide for determining the meaning and scope of this fundamental constitutional guarantee. The decision in Perez, although rendered by a deeply divided court, is a judicial opinion whose legitimacy and constitutional soundness are by now universally recognized.
…we conclude that, under this state’s Constitution, the constitutionally based right to marry properly must be understood to encompass the core set of basic substantive legal rights and attributes traditionally associated with marriage that are so integral to an individual’s liberty and personal autonomy that they may not be eliminated or abrogated by the Legislature or by the electorate through the statutory initiative process.
…our state now recognizes …that an individual’s sexual orientation — like a person’s race or gender — does not constitute a legitimate basis upon which to deny or withhold legal rights.
There’s lots good to read, and I just got to the first 10 pages.