As the Supreme Court hears arguments this week on California's Proposition 8 and the Defense of Marriage Act, Tolman is siding firmly with the opposition to both and for same-sex marriage.
It's one thing for a private institution, like a church, to prohibit gay marriages, Tolman says, "but for the government to pass a law that indicates you will never be equal, that's very difficult."
When his sister Dayna came out as lesbian in the mid-1990s, Tolman thought gay marriage was a "foreign concept," and he wasn't sure the government should sanction it. Time and personal experience have changed his mind.
Tolman's journey to support same-sex marriages parallels that of others in the Republican Party and political leaders whose views have changed in recent years.
As the former Republican legal counsel to the Senate Judiciary Committee, Tolman struggled to pair his conservative and legal moorings against his sister's fight to wed Tammy, her partner of decades.
"I've been able to see that process up close and personal," Tolman says. "We always joke in my family that Dayna and Tammy have the best marriage. Yet that joke has always been somewhat tongue-in-cheek."
Until Friday, that is.
Dayna Tolman, who lives in Washington state, wed Tammy Snow at a Seattle courthouse Friday, exactly 29 years after the two first realized they were in love. Their adopted son, Tre, held the rings.
"Tammy put that ring on me, and, I can't take it off," says Dayna Tolman, who bought the ring seven years ago but wouldn't wear it until they were legally wed. "This means something now."
Brett Tolman wasn't able to make the official nuptials but notes it's been a long time since Dayna and Tammy held an informal ceremony in an American Fork basement, where their cat, Jazz, was the ring bearer.
Tolman, who co-wrote a friend-of-the court brief for the Utah Pride Center backing gay marriage, waited in line Tuesday to attend the Supreme Court arguments but wasn't able to nab a ticket. He'll be back Wednesday in hopes of seeing the second case.
"This is a crossroads for our generation," says Tolman, who resigned as U.S. attorney in 2009 and is now in private practice. "I've often asked myself, 'What kind of lawyer would I have been at an historical crossroads, [such as] fighting for African-American rights and equality?' This was my moment and my family's moment."
More and more Americans are joining Tolman's ranks.
A CNN/ORC International Poll out this week shows 53 percent favor gay nuptials, an increase from the 40 percent who backed the unions in 2007.
Utahns' attitudes are changing, too.
In 2004, 54 percent of Utah voters said there should be no legal recognition of gay unions, according to the Utah Colleges Exit Poll. And 29 percent opposed gay marriage in 2012.
Utah exit polls, by Brigham Young University's Center for the Study of Elections and Democracy, show 72 percent of Democrats now favor gay marriage, while 53 percent of Republicans and 48 percent of independent voters back civil unions.
None of Utah's federal officials backs gay marriage nor any top state leaders. A majority of voters backed an amendment to the Utah Constitution in 2004 to bar same-sex marriage.
But Republican Party Chairman Reince Priebus told USA Today this week that while the GOP will adhere to its platform which opposes gay marriage there is a way to strike a balance between "principle and grace and respect."
Republican strategist Ana Navarro put it more bluntly.
"There is no putting this genie back in the bottle," she said Sunday on CNN's "State of the Union." "It is undeniable. The shift is here, and we're not going back."
Jon Huntsman, former Utah governor and ex-presidential candidate, has endorsed same-sex marriage as has Ohio Republican Sen. Rob Portman, who noted his son, Will, recently told him he was gay and helped prompt the conversion.
Tolman says having a close relative in this case, a sister he idolized growing up who is gay and, under the law, unequal, pushes the issue from abstract to reality.
"I get emotional about it," Tolman says. "I love my sister and know the challenges she's gone through. To hold their son, to see the love they have in their home, to feel the compassion, to feel the affection, I don't know how you can be human and not be moved."