United Kingdom, Sept. 23 -- Penny Lancaster would have left Rod Stewart if he refused to have children.

The 54-year-old model has been married to the 80-year-old rock star for 18 years, and she has admitted she needed him to be open to expanding his family if their relationship was ever going to work.

Asked if she'd have broken up with him if he remained against the idea of more children, she told Saga Magazine: "Yes.

"And that would have been very hard. But we just went with the flow."

Rod has Sarah Streeter, 60, with ex-girlfriend Susannah Boffey, Kimberly, 45, and Sean, 44, with actress Alana Stewart, Ruby, 37, with ex-girlfriend Kelly Emberg, and Renee, 32, with ex Rachel Hunter.

After the Maggie May singer changed his mind about...