Phillips replied he couldn't, but that he'd be glad to make one for other occasions, such as birthdays. Left unsaid was how making a gay wedding cake would violate his Christian faith, how he does not make ones for Halloween or bachelor parties, either.
Craig and Mullins left the shop, stunned. Left unsaid was how they viewed themselves as a regular couple, their wedding a private celebration, not a political statement. They simply wanted a no-frills cake.
Crushed, they posted a note about the encounter on Facebook and soon the cake had become a cause, with the sides becoming stand-ins for the culture wars: Phillips was the intolerant business owner. The couple became the gay rights activists pushing their agenda.
It was one of several incidents that inspired legislation in at least 11 states that would have allowed business owners to cite their religious beliefs in denying service to patrons. Most have died amid a national outcry that they would legalize discrimination.
Along the way, the stories of those caught up in the clash over fast-changing social mores can get lost. Phillips, Craig and Mullins are just three.
Phillips, 57, grew up near the teeming strip mall that houses his bakery in Lakewood, on the edge of Denver's suburban sprawl.
After graduating high school, Phillips went to work at a bakery and found he enjoyed the adrenaline kick and sense of achievement that came from catching doughnuts as they came off the conveyor belt and glazing and sprinkling them.
Nowadays, he loves his work for the way it lets him improve people's lives. "That's," he said, "what Christ does."
Phillips grew up in a religious household, but in his early 20s he felt adrift. He drank and fathered two children with his girlfriend Debbie. As he was driving one day, he felt something extraordinary. "The Holy Spirit convicted me of my sin," he recalled.
Shaken, he told Debbie that night he had found Christ. She said the same had just happened to her. They married and had a third child.
Eventually Phillips started his own shop, serving residents of the new housing developments that were rising nearby. His daughter and 87-year-old mother also work there now.
From the start, he knew there'd be limitations on what he could do. "In everything I do, I think about how people will perceive Christ through me, by what I sell, what I make," Phillips said.
The display cases bulge with cakes of every color. One depicted a trio of crosses on a hill, with the words "He Has Risen."
Phillips takes his cake-making personally. As he prepares a cake for a child's first birthday, Phillips makes a separate cupcake-sized piece to be placed on the kid's high chair, envisioning the moment the tot will dig into it, smearing frosting across his or her face.
For weddings, he interviews the couple to find out how they met, their mutual interests, what color dresses the bridesmaids will wear.
"When I decorate a cake, I feel like I'm part of the party," said Phillips, who had refused previous orders for cakes for gay weddings.
Phillips said he once employed a gay man in his bakery and makes regular birthday cakes for a lesbian couple. His youngest daughter, Lisa Eldfrick, 34, said Phillips never had problems with her and her siblings' various gay friends.
Since Mullins and Craig visited in July 2012, gay marriage has been legalized in nine states. Polls show that a majority of Americans now approve of it. Phillips is unruffled.
"The Bible says it's going to happen," he said. "This is a sign we don't acknowledge Him as our Creator."
Dave Mullins and Charlie Craig were sitting on their couch one Sunday morning in 2012, watching television. Craig turned to Mullins and said: "We should get married."
"Just so you know," Mullins replied, "I have a ring on hold for you."
When they were growing up, gay marriage was a foreign concept.
Craig, 34, grew up in a Wyoming town where he was viciously teased, even as he tried to deny his sexuality by dating girls. He graduated high school the year Matthew Shepard, a gay University of Wyoming student, was killed because of his orientation.
Craig said he still feels awkward if Mullins holds hands with him in public. "I feel like something bad could happen to us," he said in their new town house in Denver, posters of Radiohead and Bjork on the walls.
Mullins, 29, grew up in Colorado at a time when voters passed a ballot measure to prevent any city from passing protections for gays. The U.S. Supreme Court struck down the measure, laying the legal groundwork for gay marriage rulings nearly 20 years later.
The couple met in Denver through a mutual friend. A 2006 ballot measure outlawed gay marriages in Colorado, so they planned a small wedding in Massachusetts, where it was legal. That would be followed by a larger reception in Colorado.
The Lakewood restaurant hosting the reception suggested they get their cake at Masterpiece. They took Craig's mother, visiting from Wyoming, to the shop to help pick a cake. "We wanted this just to be about us," Mullins said.
The couple said Phillips' rejection was more painful than the times they have been slurred in public.
"It's like the institution and society are saying: 'You're not equal,'" Mullins said.
Within minutes of his Facebook post about the encounter, supportive messages began arriving in Mullins' account — and Phillips' shop was deluged with angry emails and phone calls.
The American Civil Liberties Union filed a complaint on the couple's behalf. Phillips' attorney argued, unsuccessfully, that the cake was a form of political speech.
A judge in December ruled Phillips violated state law that forbids refusal of service based on sexual orientation, and ordered him to make the cake or pay a fine.
"The heart of the issue is: Am I going to obey and serve what I believe the Bible is teaching?" Phillips said.
After the ruling — Phillips is appealing — so many supporters swarmed Phillips' shop that they sold out of everything, even after frantically baking 360 chocolate chip cookies. Mullins and Craig were inundated by offers of free cakes from as far away as Japan.
In the end, they accepted one from another local bakery. In an acknowledgement of the role their wedding reception now played in the campaign for gay rights, they added a rainbow layer between the mocha and the chai.
The affair has made them realize, Mullins said, that "as a minority, you don't have the option to opt out of the culture wars."