"I called it off because I didn't think it was the right thing for me to do," she told "On-Air with Ryan Seacrest" Wednesday morning. "Hef's lifestyle isn't the most normal lifestyle. This isn't the lifestyle for me, multiple girls all around. It's not the lifestyle I wanted."
Harris, 25, and Hefner, 85, announced Tuesday that their Saturday nuptials had been called off, with Hefner tweeting that his ex-fiancée had a "change of heart." The two got engaged in December. This would have been Hefner's third marriage.
"For a while I've been having second thoughts about everything. I haven't really been at peace with myself lately. I didn't think it was really fair to him," she said in the radio interview. "It was all happening too fast for me. .... Was this what I wanted?"
Harris said Hefner understood her position and denied earlier reports of an argument. "It was mutual between Hef and I," she said. "We both agreed that it wasn't the best idea to get married. He was doing it for me because he thought it was what I wanted."
The last-minute change of plans has been made more awkward by the fact that the newest issue of Playboy refers to Harris as "Mrs. Crystal Hefner" on its July cover. (Warning: The cover image contains partial nudity so if you have delicate sensibilities, don't click on the link).
Despite the split, Harris said Hefner is doing great. "I feel happy and at peace that everything will be OK," she said. "I respect him more than anything, and we're still going to be really close."