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Mackenzie says she doesn't know if that was the first time she had sex with her father, but she knows it wasn't the last.
In 1981, Mackenzie went on the road with her father and their band, the New Mamas & the Papas. By this time, Mackenzie says her drug use had escalated to a raging cocaine habit.
"We're touring, and I begin waking up after drug-fueled events with my pants around my ankles and my father sleeping beside me," she says. "Again, [I thought]: 'Don't think. Don't look. Just keep going.' And this happened over time. It didn't happen every day. It didn't happen every week, but it certainly happened."
John's secret sexual relationship with his daughter lasted for 10 years.
"It became a consensual relationship over time, and I know that I can't be the only one this has happened to," Mackenzie says. "Nobody's talking about this, and someone needs to put a face on not only nonconsensual incest but consensual incest, because I know it exists."