A law allowing same-sex couples to adopt children was upheld by the Mexican Supreme Court in a 9-2 vote Monday.
Protesters of the law argue that children are being denied their right to having a traditional family.
Gay rights advocates, however, say that the preferences of the parents don't impact a child's sexual orientation. Nine of the court's 11 justices agreed.
"There is no reliable evidence that sexual orientation determines, by itself" any other type of behavior," Justice Arturo Saldivar said, adding "the preferences of the parents do not determine (a child's) sexual orientation ... that is a discriminatory argument."
However, the court's dissenters, Chief Justice Guillermo Ortiz Mayagoitia and Justice Salvador Aguirre Anguiano, said the institution of marriage existed prior to Mexico's Constitution.
They added that traditional marriage permanently joins a man and woman with the willingness to procreate - something that would be broken should gay adoption be allowed to go forward.
Monday's ruling comes after a same-sex marriage law was passed in Mexico in March allowing married couples to adopt children, have joint bank loans, inherit wealth, and be covered by their spouses' insurance policy.
"Today, institutionalized homophobia has been buried," said Jaime Lopez Vela, a leader of the group Lesbian, Bisexual, Gay, Transsexual and Transgender Agenda. "We are happy, because now we have the same rights and responsibilities of any other married couple."