How Lily Allen's Daughters Inspired David Harbour to Marry the Singer During the Pandemic

David Harbour made it official with Lily Allen due, in part, to some accidental advice from the British singer's two young children.

The Stranger Things star, 46, appeared on Jimmy Kimmel Live! Tuesday, where he spoke about how Allen's daughters Marnie Rose, 8, and Ethel Mary, 9, whom she shares with ex Sam Cooper, played a role in the couple's decision to wed in a low-key Las Vegas ceremony last September amid the coronavirus pandemic.

"We got engaged right before the pandemic. Then the pandemic hit and nothing was happening," Harbour said of his plans with his partner, 36.

"We were riding around beginning of pandemic — she has two kids, these beautiful daughters, 8 and 9 — and we were riding bikes out in the country trying to figure out what we were gonna do for the next year or however long it was going to take. And the little one was riding along and she was going like, 'David, dad, David, dad.' Because the D got her confused," he recalled. "And the older one got very upset with that, which I understand. She was like, 'He's not our dad! He's not our dad!' "

Harbour continued: "And then younger one was like, 'Well what is he? He's kind of our dad.' She's like, 'No he's not, he's our stepdad.' And she goes, 'What is he?' And she goes, 'He's just some guy in our lives!' "

According to the Black Widow actor, that conversation confirmed it was time for him to take the next step.

"I was like, 'I need to marry this woman,' " Harbour joked. "Because the emotional fallout ... "

Although he's officially been a doting stepdad to Marnie and Ethel for months now, Harbour told Jimmy Kimmel that the girls will still jokingly call him "some guy in our lives."

"That's one of my favorite things I've ever been called," he said. " 'Just some guy in our lives.' 9 years old going on 35. 'Just some guy in our lives.' "

RELATED VIDEO: David Harbour and Lily Allen Are Married! See Photos from Their Las Vegas Wedding

The actor and the "Smile" singer tied the knot on Sept. 7 in Las Vegas, in a low-key ceremony officiated by an Elvis Presley impersonator and followed by a meal of In-N-Out burgers.

The wedding came one day after the couple obtained a marriage license, which was leaked online in a Twitter post by Clark County, Nevada, at the time.

"Clearly what happens in Vegas does not stay in Vegas," Harbour told Kimmel of the ordeal. "It's outrageous, right. We went and registered the night before and then it broke. We were just trying to get in and out real quick."

Harbour — who was first romantically linked to Allen in January 2019 — sang his wife's praises in conversation with PEOPLE in December.

"I married a beautiful, incredible woman who I love," the Hellboy actor said. "I have never met someone who is so deeply kind as her. I've never felt so taken care of and cared for by another human being."

"She really is just a deeply kind individual and I love that about her," he added.