BOSTON, Mass. (AP) — The Supreme Judicial Court is seeking legal briefs from “interested persons” as it considers an advisory opinion on whether establishing civil unions for same-sex couples — giving them the rights of married couples without the title — would meet the mandates of the court’s recent gay marriage decision.

Last week, the state Senate gave preliminary approval to a bill that would legalize Vermont-style civil unions in Massachusetts, then sent the legislation to the SJC for an advisory opinion on whether such a solution would pass the court’s muster.

Advocates for and against gay marriage intend to file briefs, and aides for Gov. Mitt Romney and Attorney General Thomas Reilly are considering doing so.

“This is encouraging to me,” said Ron Crews, president of the Massachusetts Family Institute, which opposes the SJC decision lifting the state’s gay marriage ban. “I’ve never seen this as a settled matter.”

The state’s high court issued a landmark decision in mid-November ruling that it was unconstitutional to bar gay couples from marriage. The opinion, however, gave the Legislature 180 days to act as it “deems appropriate” before the decision takes effect.

“We believe this majority opinion is a weak majority,” Crews said of the SJC’s 4-3 decision. “I don’t think it is strong in its absolute conviction that the Legislature does not have the right to define marriage.”

But Harvard Law School professor Laurence Tribe said the SJC’s request for briefs should not be read as an indication that the case is still open, or that the court will back away from the decision.

“It’s certainly not unprecedented and, in a case of this kind, anything less might have struck many people as signaling an unwillingness to hear people out,” he said.

Gay marriage advocates said they would weigh in with briefs strongly supporting same-sex marriage. They also believe the SJC’s decision was clear.

“We are very confident that the court meant what it said, that only civil marriage is equality and the civil unions are clearly a second-class status,” said Arline Isaacson, co-chairwoman of the Massachusetts Gay and Lesbian Political Caucus.