PORTLAND — Maine’s Catholic church teaches that marriage is the lifelong, exclusive union between a man and a woman, but that’s not stopping a group of Catholics from speaking out in favor of same-sex marriage.

Catholics for Marriage Equality says it’s placing quarter-page ads in three Maine newspapers Sunday urging residents to vote Nov. 6 to make same-sex marriage legal. More than 100 Catholics are attaching their names to the ads.

Anne Underwood, who co-founded the group in 2009, said most Catholics support gay marriage. In a survey by The Pew Forum on Religion and Public Life this summer, 58 percent of Catholics said they favored same-sex marriage, with 33 percent opposed.

“We support marriage for same-sex couples because it’s a matter of conscience,” said Underwood, a Topsham lawyer. “And Catholics have an obligation to form their own consciences, especially on political issues and issues of morality.”

In a prepared statement Thursday, the head of Maine’s diocese said that Catholics who support same-sex marriage are unfaithful to Catholic doctrine, and that Catholics for Marriage Equality and other dissident groups don’t speak for the church.

“A Catholic whose conscience has been properly formed by Scripture and church teachings cannot justify a vote for a candidate or referendum question that opposes the teachings of the church,” Bishop Richard Malone said.

Maine’s Catholic diocese took an active role in the 2009 campaign opposing gay marriage. Mainers voted 53 percent to 47 percent to overturn a gay marriage law passed by the Legislature.

This time, the Catholic church isn’t campaigning against the ballot measure or contributing money to groups that are campaigning against it. Rather, it is focused on teaching parishioners about what it calls the sanctity of marriage between a man and a woman.

Underwood helped form Catholics for Marriage Equality in 2009 because she was disappointed with the church’s activities in that campaign.

The group held a spaghetti dinner in Bangor this month to draw attention to its cause while raising money for homeless shelters. Former Democratic Gov. John Baldacci cooked the spaghetti and dished it out. A similar fundraiser is planned Friday in Portland.

Baldacci, who publicly supported gay marriage in 2009 when he was governor, said others in his faith who share his view on the issue are religious people who believe in the Catholic Church.

“We also recognize that God gave us the ability of free choice and to be able to follow our hearts,” Baldacci said.

Gay marriage opponents in Maine say they have received a $100,000 contribution from the Knights of Columbus, a Catholic charitable organization. The Protect Marriage Maine political action committee says the money came from the Knights of Columbus’ international offices in New Haven, Conn.