"She wanted marriage ... children," the cyclist says. "That pressure cracked it"

Lance Armstrong is finally revealing why his engagement to musician Sheryl Crow ended three years ago.

In Lance, a book due out in July, Armstrong tells author John Wilcockson the decision to have children became an issue in their relationship.

“She wanted marriage, she wanted children; and not that I didn’t want that, but I didn’t want that at that time because I had just gotten out of a marriage, I’d just had kids,” he says. “Yet, we’re up against her biological clock – that pressure is what cracked it.”

The Tour de France champ says he and Crow underwent counseling to address their problems but it didn’t work. “We were at different points in our lives. We were not compatible on that issue.”

Crow went on to adopt a son, Wyatt, in May 2007. And last December, Armstrong announced that he and his girlfriend Anna Hansen are expecting a baby in June.