Ashton Kutcher and Mila Kunis had an "agreement" to keep their relationship purely sexual when they first got together.
The 39-year-old actor and the brunette beauty - who have two-year-old daughter Wyatt and six-month-old son Dimitri together - began seeing one another in 2012 after first meeting on 'That 70s Show' in the 1990s, and Ashton has now revealed their relationship started a simple hook-up.
Ashton even drew similarities between his 2011 movie 'No Strings Attached' and Mila's 'Friends With Benefits', and said their relationship followed a similar plot to both films.
He revealed: "She's like, 'You're not ready to be in a relationship' and I'm like, 'I'm not ready to be a in a relationship.'
"It was funny because I'd just done this movie called No Strings Attached and she just did a movie called Friends With Benefits and we legitimately lived out our movies, which was virtually the same movie. At the time, we were like, 'OK, we have an agreement.'"
And the 'Jobs' star - who was previously married to Demi Moore - also revealed they started to get intimate when Mila, 33, wanted to breathe in the smoke from his cigarette.
Speaking on 'The Howard Stern Show' on SiriusXM, Ashton said: "I was smoking cigarettes at the time and I was still a smoker and she had quit smoking and she wanted me to like shotgun cigarette smoke so she could breathe it in. And I was like, 'Well alright.' And so I started doing this and gradually through the night, it got closer and closer and closer. I think it was pretty mutual - it was kind of obvious that things were happening."
After getting together in 2012, Ashton and Mila became engaged in 2014, and wed one year later in 2015.