BOSTON – The high court that legalized same-sex marriage in Massachusetts has agreed to hear a challenge to the 1913 law used to bar out-of-state gay couples from getting married here.

The law denies out-of-state couples the right to marry if the union would be illegal in their home state. Since Massachusetts is the only state that allows gays to marry, state officials used it to block nonresident couples from being issued marriage licenses.

Eight couples – from Connecticut, Rhode Island, New Hampshire, Vermont, Maine and New York – sued last June, claiming the law was discriminatory. Clerks in 13 communities filed a companion suit, saying they were being forced to discriminate.

In August, a Superior Court judge upheld the law, ruling it wasn’t discriminatory because the state was applying it evenly to both gay and straight couples.

The plaintiffs asked the state appeals court to hear the case, but the Supreme Judicial Court decided last month to reach down and take the case. The direct review by the court, which ruled in November 2003 that gay couples should be allowed to marry in Massachusetts, bypasses the usual review by the state Appeals Court.

Oral arguments are scheduled for September, court spokeswoman Joan Kenney said Wednesday.

“We’re optimistic about our chances because the court has already decided the commonwealth can’t deny marriage rights to gays and lesbians,” said Michele Granda, an attorney for the Gay & Lesbian Advocates & Defenders, which represents the eight couples.

A spokesman for Attorney General Thomas Reilly, whose office will defend the law before the court, declined to comment on Wednesday.

State attorneys have argued that the 1913 law protects other states’ rights to define marriage as they see fit, a principle repeatedly cited by the court in its landmark gay marriage ruling. The ruling requires “an approving state” in any marriage, and that doesn’t exist outside Massachusetts, prosecutors said.

Gays began marrying in Massachusetts last May. Gov. Mitt Romney invoked the 1913 law and instructed city and town clerks not to issue licenses to nonresident gay couples.

A proposed amendment to the state constitution would ban gay marriage but allow civil unions. But it requires another round of legislative approval and the earliest it can go before voters would be November 2006.