BOSTON (AP) – State Attorney General Thomas Reilly, defending a 1913 state law prohibiting out-of-state couples from marrying in Massachusetts if their weddings are prohibited in their home states, argues in court documents that the nearly century-old law is not being used to discriminate against same-sex couples.

The 1913 law has been hotly debated since May 17, 2004, when Massachusetts became the first state in the country to allow gay marriages.

Gay-rights advocates have argued that the 1913 law is unconstitutional and has been unfairly applied to out-of-state same-sex couples since Massachusetts began allowing gay couples to marry here. The law is being challenged in a lawsuit filed on behalf of eight same-sex couples from six other states who wanted to get married in Massachusetts but were denied.

The group Gay and Lesbian Advocates and Defenders, or GLAD, argued in court documents filed in March that the state is using the 1913 law as a “weapon of purposeful discrimination.”

But Reilly, in defending the law, argues in court documents filed Friday that the law is being applied evenly – to both heterosexual and gay couples – and is not being used in a discriminatory fashion. He also argues that other states have the right to their own definition of marriage.

The 1913 law respects “the marriage impediment laws of other jurisdictions, whatever those laws might be, with regard to both same-sex and opposite-sex couples,” Reilly argued in a 165-page brief written by Assistant Attorney General Peter Sacks.

GLAD, which filed the lawsuit on behalf of couples from Connecticut, Maine, New Hampshire, Rhode Island, Vermont and New York, has argued that the state Supreme Judicial Court’s landmark ruling that paved the way for gay marriage in Massachusetts also applies to out-of-state residents who want to marry here.

GLAD argues that the only couples who would not qualify for marriage in Massachusetts are those who live in states that expressly prohibit same-sex weddings.

Same-sex marriage advocates have argued that the old law was resurrected and enforced in a discriminatory way as part of an effort to stop gay marriages.

But Sacks told reporters Friday that the law has been invoked for both same-sex couples and heterosexual couples.

“This statute says whether you’re a same-sex couple or an opposite-sex couple, if you can’t get married in your home state, you can’t get married here,” Sacks said.

In August 2004, Superior Court Judge Carol Ball upheld the 1913 law. In her ruling, Ball said she was “troubled” by the state’s decision to suddenly begin enforcing the law after gay marriage became legal in Massachusetts. But she said the law was being equally applied and was not discriminatory because the state has a reason to ensure that marriages approved in Massachusetts are valid in other states.

The Supreme Judicial Court, which ruled that the Massachusetts Constitution gives same-sex couples the same right to marry here as heterosexual couples, is scheduled to hear oral arguments in the case in October.

Republican Gov. Mitt Romney, an opponent of gay marriage, invoked the 1913 law to bar gay couples from other states from getting married in Massachusetts.

Some municipal clerks defied Romney and issued licenses to anyone who applied. But Reilly, acting on Romney’s instructions, ordered them to stop.

Reilly, in his legal brief, said the governor’s stance on same-sex marriage is irrelevant. Reilly said the state Registrar recognized there would be a greater likelihood of violations of the 1913 law once gay marriages became legal in Massachusetts.

“The Registrar has made every effort to do so in a manner that treats same-sex and opposite-sex out-of-state marriage applicants evenhandedly,” Reilly’s brief said.

Editors’ Note: Denise Lavoie is a Boston-based reporter covering the courts and legal issues. She can be reached at dlavoie(at)ap.org