The couple was already engaged when the COVID-19 pandemic hit in March 2020 and, instead of waiting for the crisis to blow over to stage a wedding, the pair decided to jet out to Las Vegas in September to become husband and wife.
Allen's two girls from her marriage to Sam Cooper joined them for the low-key trip, and Harbour admits they decided to take the plunge after overhearing a conversation between Ethel, nine, and Marnie, eight.
During an appearance on America's "Jimmy Kimmel Live!", he explained, "We were riding around (the) beginning of the pandemic - she has two kids, these beautiful daughters, eight and nine - 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!' "
"And then the younger one was like, 'Well, what is he? He's kind of our dad.' (Ethel)'s like, 'No, he's not, he's our stepdad... He's just some guy in our lives!' "
The remark made Harbour realise he and Lily needed to make their relationship official sooner rather than later. "I was like, 'I need to marry this woman...,' " he quipped.
And even though the couple has now been married for almost nine months, Ethel and Marnie will still occasionally joke about Harbour being "just some guy in our lives," using the phrase as a term of affection.
"That's one of my favourite things I've ever been called," Harbour smiled. " 'Just some guy in our lives.' (Ethel is nine) years old going on 35. 'Just some guy in our lives!' "