A new legal battle taking shape in Montana is challenging the state on domestic partnerships, not marriage, for gay and lesbian couples.

The first-of-its-kind lawsuit was filed Thursday by seven gay couples who are being represented by the American Civil Liberties Union (ACLU).

The ACLU is suing the state for domestic partnerships because Montana has a constitutional amendment that defines marriage as a heterosexual union.

In its complaint, the ACLU wrote that denying gay couples of equal protections afforded to heterosexual couples “constitutes unconstitutional discrimination based on sexual orientation.”

“It's unfair for same-sex couples who have made commitments and formed families to be treated by the state like legal strangers,” Betsy Griffing, legal director for the ACLU of Montana, said.

“Lesbian, gay and bisexual Montanans are valuable and productive members of society who should be treated fairly if their partner is in the hospital or dies without a will,” she added.

Several states, including California, Washington and most recently Nevada, have approved domestic partnership laws that give gay couples most of the rights and obligations associated with marriage.

A similar lawsuit is moving forward in Hawaii, where gay couples represented by the gay rights group Lambda Legal are suing the state after Governor Linda Lingle vetoed a gay-inclusive civil unions law. Hawaii also has a gay marriage ban in place.