BOSTON (AP) – A 1913 law the state is using to block same-sex marriages by nonresidents should apply only when the couples are from states that expressly forbid the practice, gay-rights advocates said in a brief filed with Massachusetts’ highest court.

The law, which bars marriages in Massachusetts if a couple’s home state doesn’t sanction the union, has been in dispute since the nation’s only state sanctioned same-sex weddings began taking place last May.

The Supreme Judicial Court, which ruled that the state constitution requires that same-sex couples be allowed to marry, agreed last month to hear the case over the 1913 law. It’s being challenged by eight couples – from Connecticut, Rhode Island, New Hampshire, Vermont, Maine and New York – whose attempts to get married in Massachusetts were rejected.

Gay & Lesbian Advocates & Defenders, which filed the brief on behalf of the couples, argued that the state is using the law as a “weapon of purposeful discrimination.”

“As it currently stands, this [law] should only block couples who come from states where their marriage would be void, where a statute expressly says same-sex couples’ marriage would be void,” GLAD attorney Michele Granda said. “In the absence of that, there is nothing that prevents them from marrying here.”

The state Attorney General’s Office, which is defending the law, has until early May to file its brief in the case. The state has argued that the 1913 law protects other states’ rights to define marriage as they see fit.

A Superior Court judge upheld the law in August, saying it was being equally applied and was not discriminatory.

The Supreme Judicial Court is expected to hear oral arguments in September.

Massachusetts is the only state that allows same-sex marriage, although Vermont allows civil unions and local officials in parts of California, Oregon and New York allowed same-sex weddings early last year until courts stopped them.

Voters in 13 states responded to the trend last year by approving bans on same-sex marriage, and states including Kansas, South Dakota and Alabama have similar proposals on upcoming ballots.

A proposed amendment to the Massachusetts Constitution would ban same-sex marriage but allow civil unions. The earliest residents could vote on it is November 2006.