## Facts

- Mexico City’s legislature amended its Civil Code in December 2009 to define marriage as the “union of two persons” and to allow adoption by married couples without regard to the spouses’ sex.
- The Federal Attorney General filed an action of unconstitutionality in January 2010 seeking to invalidate the amended marriage and adoption provisions.
- The challenge argued that constitutional protection of the family required a heterosexual marital model; that local lawmakers lacked authority to redefine marriage; that existing domestic-partnership law avoided discrimination concerns; and that same-sex joint adoption conflicted with the best interests of the child.
- The Supreme Court of Justice of the Nation (en banc) heard the action and issued its main rulings in August 2010, later issuing a formal resolution rejecting the challenge and sustaining the provisions’ validity.
- In a related subsequent ruling, the Court required all Mexican states to recognize marriages validly celebrated in Mexico City.

## Issues

1. Whether Mexico City’s gender-neutral definition of marriage violates constitutional protections concerning equality, non-discrimination, and protection of the family.
2. Whether allowing same-sex married couples to adopt jointly violates constitutional norms and the best interests of the child.
3. Whether local civil-law authority permits Mexico City to define marriage in gender-neutral terms, or whether marriage is constitutionally fixed at the federal level.
4. Whether marriages validly celebrated in Mexico City must be recognized by other Mexican states.

## Decision

- The Court upheld the constitutionality of Mexico City’s marriage amendment, rejecting the claim that the Constitution confines marriage or family to a heterosexual model.
- The Court upheld the constitutionality of allowing joint adoption by same-sex married couples, holding that exclusions based on sexual orientation lacked constitutional justification.
- The Court concluded that local authorities have competence to regulate civil marriage and that the Constitution does not bar a gender-neutral marriage definition.
- In the related subsequent ruling, the Court held that other states must recognize marriages validly celebrated in Mexico City under the constitutional rule of inter-jurisdictional recognition of civil acts.

## Legal Principles

- Sexual-orientation classifications are treated as suspect and trigger rigorous review; the state bears the burden to justify differential treatment with strong reasons supported by evidence.
- Constitutional protection of the family is not limited to a single, heterosexual, procreative model; constitutional text does not define family in exclusive terms.
- Local civil-law competence includes defining and regulating civil marriage, absent a specific federal constitutional restriction.
- Best interests of the child govern adoption decisions; categorical exclusion of prospective adoptive parents based on sexual orientation is not justified without proof of harm and cannot replace individualized determinations.
- International human rights sources may be used to confirm constitutional equality and non-discrimination norms in assessing restrictions on family life and parental status.
- Valid marriages celebrated in one jurisdiction must be recognized across the federation under constitutional rules governing recognition of civil-status acts.

## Conclusion

The Court sustained Mexico City’s reforms opening civil marriage and joint adoption to same-sex couples, reasoning that constitutional family protection is plural, sexual-orientation-based exclusions are presumptively invalid, and adoption must be assessed by the child’s best interests on a case-by-case basis; it also required nationwide recognition of Mexico City marriages.


Action of Unconstitutionality 2/2010 (Sup. Ct. Justice of the Nation (Mex.) 2010)