Gay marriage in Mexico unconstitutionally banned, says that country's supreme court — unanimously

Last December, Mexico's highest tribunal ordered the state of Oaxaca to recognize the marriages of the three same-sex couples that had filed suit, but it didn't release its written justification until today's unanimous opinion, perhaps because of disagreement on how broad its opinion should be.
     2013 Alicia Patterson Fellow J. Lester Feder, author of the aftermarriage.org blog, explained today's ruling for Buzzfeed Politics:

     Denying same-sex couples the right to marry is unconsti­tutionally discriminatory, Mexico's Supreme Court announced in a sweeping ruling made public Monday.
     ...Writing for a unanimous tribunal, Minister Arturo Zaldívar Lelo de Larrea invoked the U.S. cases Loving v. Virginia and Brown v. Board of Education to argue for marriage equality in a way that American activists would be overjoyed to see from a justice of the U.S. Supreme Court.
     On Loving v. Virginia, which struck down laws against interracial marriage in 1967, Zaldívar wrote (translated from its original Spanish):

The historical disadvantages that homosexuals have suffered have been well recognized and documented: public harassment, verbal abuse, discrimination in their employment and in access to certain services, in addition to their exclusion to some aspects of public life. In this sense … when they are denied access to marriage it creates an analogy with the discrimination that interracial couples suffered in another era. In the celebrated case Loving v. Virginia, the United States Supreme Court argued that "restricting marriage rights as belonging to one race or another is incompatible with the equal protection clause" under the US constitution. In connection with this analogy, it can be said that the normative power of marriage is worth little if it does grant the possibility to marry the person one chooses.

So much for the ridiculous argument that gay people are not excluded from marriage since they, too, have the right to marry opposite sex partners!
     Zaldívar also compared the discriminatory principal of separate-but-equal, discarded in the U.S. 1954 school desegregation case Brown v. Board of Education, to laws restricting same-sex couples to civil unions or domestic partnerships while barring them from marriage and cited a Chilean case:

     The court broke important ground in the ruling by invoking another precedent from international law, a ruling handed down in 2012 by the Inter-American Court of Human Rights, Karen Atala Riffo y Niños v. Chile. Most Latin American countries recognize the jurisdiction of the Inter-American Court, which is the legal arm of the Organization of American States. (The U.S. and Canada do not submit to the court's jurisdiction.)
     Karen Atala was a Chilean mother who was denied custody of her children during divorce proceedings with her ex-husband because she is a lesbian. The Inter-American Court said the Chilean courts violated Atala's human rights and for the first time said that gays and lesbians were protected from discrimination under international law...

The Atala precedent could open the door to marriage equality throughout Latin America. In Mexico, there are still loose ends:

...Unlike in the United States, it takes more than one ruling from Mexico's Supreme Court to strike down a law—the court must rule the same way in five separate cases before a law falls. This ruling concerns three separate cases; it will take two more for any same-sex couple in Oaxaca to be able to wed easily, and then the process may have to be repeated in other states. But this precedent means this is a procedural issue, not a legal one.