Robbins, 58, and Sarandon, 70, split in December 2009 after having been in a relationship since meeting on the set of 1988's Bill Durham. They share two sons together -- 28-year-old Jack and 25-year-old Miles.
The Oscar-winning actress opened up about never marrying Robbins in a candid interview with The Telegraph.
"I thought that if you didn't get married you wouldn't take each other for granted as easily," she explained. "I don't know if after twentysomething years that was still true."
She also talked about fans' reactions to their breakup after more than 20 years together.
"People were coming up to me in the street and saying, 'I cried and cried when I heard,'" she recalled. "Well, I was sadder! I didn't think it would ever happen, either. You bring people into your life at certain times. Maybe you have a relationship to have children and you realize that it's fulfilled after that point."
"I'm a serial monogamist," she told PrideSource. "So, I haven't really had a large dating career. I haven't exactly been in the midst of a lot of offers of any kind. I'm still not! I don't know what's going on! But I think back in the '60s it just was much more open."