SALTILLO, Mexico – Gabby and Ana are in love. So are Marco Antonio and Juan Carlos. Under a sweeping new law allowing same-sex couples to form civil unions, they are planning to turn their love into a legal commitment. Legislators in the dusty northern border state of Coahuila have stunned Mexico by giving same-sex couples property and inheritance rights long reserved for married heterosexuals. It is a first for this predominantly Roman Catholic country, a measure so swift and so bold that it surprised even the gay community. All of a sudden, gay rights are on the national agenda. Since...