BOSTON — The same court that made Massachusetts the first state to legalize gay marriage ruled yesterday that a proposed constitutional amendment to ban future same-sex marriages can be placed on the ballot, if approved by the Legislature.

The ruling was the result of a lawsuit brought by gay-rights supporters who argued that Attorney General Tom Reilly was wrong to approve the question, saying that the state constitution bars any citizen-initiated amendment that seeks to reverse a judicial ruling.

In a unanimous decision, the Supreme Judicial Court said the constitution does not bar citizen initiatives from making prospective changes to the constitution, even if that effectively overrules the effect of a prior court decision, because that change would not be a reversal.

The state Legislature is expected to take up the question Wednesday during a constitutional convention.

Citizen-initiated ballot questions must be certified by the attorney general and then approved by two consecutive legislative sessions. Before the marriage question could be placed on the 2008 ballot, supporters would need to win the votes of 50 lawmakers — 25% of the Legislature — in two consecutive sessions.

With a landmark 2003 ruling, the state’s highest court cleared the way for same-sex marriages to begin in Massachusetts in May 2004. More than 7,000 gay couples have married since.