Because there is a constitutional amendment in Montana barring marriage for same-sex couples, this lawsuit is not seeking marriage. The couples in the suit are seeking the protection of state-recognized domestic partnerships (i.e., a finding under the state constitution’s other provisions, such as its guarantees of equal protection and privacy, that denial to same-sex couples of the rights and benefits that accrue to married different-sex couples is unconstitutional).
https://www.apa.org/about/offices/ogc/amicus/donaldson