BOSTON (AP) – Massachusetts’ attorney general said a 91-year-old state law prevents the state from issuing marriage licenses to couples whose marriage would be illegal in their home state, an interpretation that could block gay and lesbian couples from 38 states from tying the knot in Massachusetts.

Attorney General Thomas F. Reilly said that when same-sex matrimony becomes legal in Massachusetts, it will apply only to residents of Massachusetts and couples who live in states where no law expressly defines marriage as the union of a man and a woman.

Reilly based his decision on a 1913 state law that prevents out-of-staters from getting married in Massachusetts if they are ineligible for marriage in the state where they reside.

Currently, state municipal clerks require couples seeking a marriage license to sign affidavits that their impending nuptials would be legal in their home jurisdiction.

Gov. Mitt Romney communications director Eric Fehrnstrom said that same-sex couples also will be required to sign such affidavits after May 17, the date the Supreme Judicial Court set for gay and lesbian couples to begin being married.

Thirty-eight states have Defense of Marriage Act laws that define marriage solely as an institution between a man and a woman.

Reilly’s interpretation came on a day he and the governor sparred over whether the state should ask its highest court to delay same-sex marriages until voters can cast ballots on a constitutional amendment banning them.

Since the Legislature approved a proposed amendment to the state constitution that would ban same-sex marriage but legalize civil unions, Romney has called on Reilly to ask the Supreme Judicial Court for a stay until after citizens get a chance to vote on the amendment, in November 2006 at the earliest.

Reilly declined, arguing that the court had made itself clear in its original November decision and an advisory opinion issued in February. Reilly’s office represented the state in the landmark same-sex marriage case.

“The arguments the governor makes are political arguments,” said Reilly. “The governor’s job is to implement the law of the state, and I expect him to do that.”

That left Fehrnstrom fuming.

“Under our constitution, the attorney general is the gatekeeper to the Supreme Court. By his action, he’s closed the gate on the people of Massachusetts,” Fehrnstrom said.