"Some people say, 'You gays are trying to redefine marriage,' but the definition of marriage has already changed," said Treadway, a 20-year-old student originally from Los Angeles. "Now it's between two people who love each other."
Most Britons agree. Polls show about two-thirds of people in the country back gay unions, and support is highest among the young. Britain has seen none of the large street protests against gay marriage that have taken place in France.
Same-sex marriage has been welcomed with enthusiasm by Britain's Conservative-led government. Rainbow flags went up over two government buildings Friday in what Deputy Prime Minister Nick Clegg called "a small symbol to celebrate a massive achievement." Many local authorities offered midnight weddings, competing to hold the first ceremonies.
Treadway and Adl-Tabatabai, a 32-year-old TV producer from London, will be married in front of 100 guests at London's Camden Town Hall, before emerging to the strains of "I Got You, Babe" by Sonny and Cher. Guests will travel by double-decker bus to the late-night reception, where they will enjoy drinking, dancing and that most British of dishes: curry.
It would have been unthinkable in the 1980s, when Prime Minister Margaret Thatcher's government passed a law banning schools and local authorities from "promoting" homosexuality or depicting it as "a pretended family relationship."
That law wasn't repealed until 2003. Yet when Parliament legalized same-sex marriage in July, it was by a wide margin and with the backing of Conservative Prime Minister David Cameron.
There was some heated rhetoric — mostly from traditionalists in Parliament's unelected upper chamber, the House of Lords — but ordinary Britons overwhelmingly supported gay marriage.
"What has amazed me is how much of Britain, how quickly, has moved toward backing us on this," columnist and former Conservative lawmaker Matthew Parris told Sky News on Friday.
Some cite the transitional step of civil partnership, a compromise introduced in 2005 that gave gay couples the same legal protections and rights as heterosexual married partners — but not the label of marriage.
Adl-Tabatabai said civil partnerships had helped some people overcome their opposition to gay relationships.
"Their preconceptions of gay people and what their lifestyles are like are kind of changing," he said. "They're like, 'OK, I hate to say it, but maybe you're like us.'"
The government also defused some opposition by exempting religious groups from conducting same-sex weddings unless they choose to opt in. Quakers and Liberal Judaism are among the few who have done so. The Church of England, the country's biggest faith, remains divided on the issue and does not conduct same-sex weddings.
Britain is the 15th country to legalize same-sex marriage, which is also legal in several U.S. states. But the march of gay rights is not universal — several countries, including Uganda and Russia, have recently introduced anti-gay laws.
In Britain, some argue that true equality won't have been reached until a gay couple can be married by a priest in Westminster Abbey, as Prince William and Kate Middleton were in 2011. Some gay-rights activists think embracing marriage is caving in to a heterosexist world order.
Still, Saturday feels momentous to couples including Laura Smith and Sarah Nutley, marrying at London's elegant Mayfair Library in a service that includes Britney Spears lyrics — Nutley is a fan — and quotes from "The Rocky Horror Show," which Smith loves.