The 63 former priests said they took their stand to counterbalance efforts of the Roman Catholic Church's leadership to defeat the Washington ballot initiative as a threat to religious liberty.
Washington state's Democratic-controlled legislature voted this year to legalize same-sex marriage and Governor Chris Gregoire, a Democrat and a Catholic, signed the measure into law in February.
But the law was blocked from taking effect when opponents submitted a petition to bring the matter before voters in November. A number of Catholic churches in the state, at the urging of their bishops, participated in the signature drive last spring.
Six states and the District of Columbia currently allow same-sex marriage. Nationally, several polls have found more Americans support gay marriage than oppose it, in a shift in attitude that began to be seen two years ago after decades of opposition to the idea in public opinion.
But gay marriage has so far been rejected in states where the question has been put directly to voters. A survey released last month by Public Policy Polling found 51 percent of Washington state voters supported gay marriage.
Catholic bishops in Washington state have waged their battle against gay marriage in large part with video pleas and pastoral letters.
In one such letter, released on Sunday, Bishop Joseph Tyson of the Yakima Diocese said the law would "challenge our right to educate about the unique value of children being raised by their own mothers and fathers."
The Washington State Catholic Conference, the public policy voice of the state's bishops, had no immediate comment on the actions of the former priests.
The former clergy's petition was organized by Patrick Callahan, who was a Catholic priest for 15 years and remains a Catholic churchgoer.
"Progressive-thinking Catholics need the reassurance that there is more than one authentic Catholic position," he told Reuters.
A Public Religion Research Institute poll conducted in March found 59 percent of Catholics nationwide support gay marriage, compared with 52 percent of the general public. No polling data was available on Catholic views of gay marriage in Washington state.