PORTLAND, Maine » Most Americans have never heard of Mary Bonauto. But inside the tightknit world of gay legal advocacy, Bonauto is a quiet celebrity — a lawyer and mother of twins who some say is almost single-handedly responsible for the same-sex marriage cases now pending before the Supreme Court.

"No gay person in this country would be married without Mary Bonauto," said Roberta Kaplan, who went before the justices Wednesday to argue one of the cases.

As the top civil rights lawyer for Gay and Lesbian Advocates and Defenders, or GLAD, based in Boston, Bonauto has spent more than a decade plotting a careful strategy to advance gay marriage rights. She prompted Vermont to create civil unions in 2000, won the 2003 case that made Massachusetts the first state to legalize same-sex marriage and last year persuaded a federal appeals court that the Defense of Marriage Act, which denies federal benefits to gay couples, is unconstitutional.

Yet in a quirk of fate, Bonauto is watching her life’s work this week from the court’s spectator seats.

The justices considered a Defense of Marriage Act case Wednesday, but it was not Bonauto’s, which she argued when Justice Elena Kagan was President Barack Obama’s solicitor general. Instead the court took up a similar case, Kaplan’s from New York, presumably so Kagan would not have to recuse herself.

"Am I disappointed?" Bonauto asked last week in the kitchen of her home here. "There is an element of disappointment, but I’m also incredibly excited. I feel like after all these years, you’re getting a hearing — a fair hearing — from the highest court in the land."

At 51, Bonauto is unassuming, serious and low key.

"She is not going to set a room on fire," said Dean Hara, a plaintiff in Bonauto’s Defense of Marriage Act case. "But when she is arguing, she is really somebody to listen to."

Even her opponents offer kind words, saying they appreciate her civil tone.

"She has always been the consummate professional, very courteous and gentle," said Kris Mineau of the Massachusetts Family Institute, even as he said her courtroom victories had "degraded the value of marriage."

Although her office is in Boston, Bonauto works mostly from her home in this seaport city, where she and her wife, professor Jennifer Wriggins of the University of Maine’s law school, are raising their 11-year-old twin daughters. Last year, along with actress Jodie Foster and Mary Cheney, a daughter of former Vice President Dick Cheney, the couple made the list of "The Most Powerful Lesbian Moms in America," published by the website mombian.com.

But Bonauto is too busy juggling legal briefs, homework and piano lessons to see herself as a woman making history.

"That’s not how we experience our lives," she said.

Bonauto would be the first to say she builds on the work of others. As early as the 1970s, gay couples began suing for the right to marry, inspired by the 1967 Supreme Court ruling in Loving v. Virginia, which struck down state laws banning interracial marriage. In 1983, a young Harvard law student named Evan Wolfson wrote his third-year thesis on why gays should be free to marry.

Wolfson was eventually hired by Lambda Legal, the gay advocacy group, where he joined a network of people dedicated to toppling laws that criminalized homosexual sex. Among them was Bonauto.

The daughter of a pharmacist and a preschool teacher from Newburgh, N.Y., she had come out, with some difficulty, while an undergraduate at Hamilton College. There, Bonauto was harassed over her sexual orientation — an experience, she said, that contributed to her desire to "make life better" for others.

By 1990, with a law degree from Northeastern University, she was working for GLAD in Boston. She had been there less than a week when a gay couple approached her with the idea of suing to get married. She said, no, the timing was not right.

"I would have cases of somebody who goes to a Dunkin’ Donuts and the wait person realizes it was a gay person and goes nuts," Bonauto recalled.

How could she pursue a seeming luxury like getting married, she reasoned, when gay people were discriminated against in housing, employment and adoption and harassed by the police?

In 1991, a Honolulu lawyer named Dan Foley did what Bonauto had declined to do and sued on behalf of three same-sex couples denied marriage licenses. The case prompted intense discussion in gay legal circles, recalled Wolfson, who later became Foley’s co-counsel and has since founded Freedom to Marry, a national advocacy group. Some thought marriage was "patriarchal and exclusionary," while others said the gay rights movement "was supposed to be about liberation," not joining heterosexuals. Still others, like Bonauto, worried that the legal foundation had not been laid and that a loss could lead to a gay rights setback.

She was right to worry; Hawaii voters eventually amended their Constitution to allow a ban on same-sex marriage, much as California would. So the advocates opened up what Wolfson called "a second front" in Vermont — in part, he said, because the legal and constitutional climate was more hospitable there, and in part "because we had a really terrific talent in Mary."

Vermont’s decision to create civil unions did not entirely please Bonauto, who saw it as a "separate system" for gay people. Of all her legal victories, the 2003 Massachusetts case sticks with her. She still remembers clutching the Supreme Judicial Court’s ruling in her hand, and can still recite its compelling first paragraph, including the declaration that the Massachusetts Constitution "forbids the creation of second-class citizens."

Once she had established a right to marriage, Bonauto went after the Defense of Marriage Act, which denied legally married gay couples the federal benefits that straight couples could have. Barney Frank, the former Massachusetts congressman, who is openly gay, said the move had sealed Bonauto’s reputation as a "first-rate lawyer and a first-rate strategist" who built on one victory after another.

"She’s our Thurgood Marshall," he said, referring to the Supreme Court justice who made history fighting racial discrimination.

Bonauto was at first skeptical of the other gay marriage case before the court this week, the challenge to Proposition 8, California’s ban on same-sex marriage. She feared the court was not ready to grant gay couples a broad constitutional right to marry, and indeed some justices expressed doubts Tuesday.

But public opinion has shifted so dramatically since the case was filed in 2009, Bonauto said last week, that she now believes the justices can find a way to "issue a marriage decision that the country would embrace."

She has witnessed that shift at home; last year, Maine became the first state where voters approved same-sex marriage, reversing a decision from 2009.

"The things that scared people three years ago don’t scare people now," Bonauto said.

In the run-up to Wednesday’s hearing, Bonauto focused on helping Kaplan prepare. She coordinated "friend of the court" amicus briefs and arrived in Washington early to participate in moot court briefings.

Kaplan said it "makes me crazy" that people do not know Bonauto’s work. Like Frank, she drew an analogy to Marshall.

"She conceived of a strategy just like him, over a long number of years, and then implemented it," Kaplan said. "It was strategically brilliant, and she succeeded. No one else can say that."

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