"I was smoking cigarettes at the time -- I was still a smoker," Kutcher told host Howard Stern. "She had quit smoking and she wanted me to shotgun cigarette smoke so she could breathe it in. I was like, 'Well, alright.'"
"I started doing this and gradually through the night, it got closer and closer," he shared. "It was pretty mutual. It was kind of obvious that things were happening."
Kutcher and Kunis played Michael Kelso and Jackie Burkhart during That '70s Show's eight-season run from 1998 to 2006. The pair kissed on-screen while Kunis was just 14, but didn't start dating until 2012.
"I think I was her first kiss on the show. We have our first kiss, like, memorialized on the TV show," the actor acknowledged.
"It was really weird. I was like, 'Isn't this illegal?'" he recalled. "It was really awkward. She was 14! She was like my little sister."
Kutcher and Kunis married in 2015, and share 2-year-old daughter Wyatt and 6-month-old son Dimitri. The actor said last week on On Air with Ryan Seacrest that they changed their son's name at the "last minute."