The 61-year-old actress fell for her ‘Look Who’s Talking’ co-star on the set of the movie almost 25 years ago but couldn’t act on her feelings because she was married to actor Parker Stevenson at the time.
In an interview with Barbara Walters, Kirstie said: "Believe me, it took everything that I had, inside, outside, whatever, to not run off and marry John. And be with John for the rest of my life. [He was the] greatest love of my life."
John, 58 – who is married to Kelly Preston – was single when they met and when Kirstie was asked why she didn’t leave her husband for her co-star, she insisted the institution of marriage was very important to her.
The actress – who divorced Parker in 1997- said: “I feel like when you marry someone you're supposed to work hard at it, and you're supposed to make it work."
Kirstie has always been open about her love life and admitted she had an insatiable sexual appetite but her marathon romps still couldn't bring her happiness in her relationships.
She has said: "The way I used to pick guys was so shallow. Hot? OK, that'll be my husband. But here's my big revelation. I decided that in the best of times you have two hours of sex a day, so you have 22 hours left."