In the 2011 romantic comedy No Strings Attached, Ashton Kutcher plays ladies' man Adam who makes an agreement to have non-committal, casual sex with a friend. But just months later he entered his own "purely sexual" pact.

The Hollywood actor has revealed his love story with wife Mila Kunis, whom he first met on the set of sitcom That 70s Show, started off as a desire to satisfy their carnal cravings.

During an appearance on the Howard Stern show, the 39-year-old star said it was purely sexual, drawing similarities between his 2011 movie with Natalie Portman and Kunis' Friends With Benefits.

Kutcher and Mina Kunis started dating in 2012 after the former castmates reconnected following the collapse of his marriage to Demi Moore.

He explained: "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.'"

He went on to reveal the moment he knew it was serious. "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."

The couple tied the knot in July 2015 and are now parents to daughter Wyatt and a son named Dimitri.