Hefner said he was the one that suggested the 20-year-old twins move out of the mansion, which will take place within the next couple of weeks.
"When they thought about it, they thought it was a good idea," he told E! News. "They will now be friends, but they won't be defined as the girlfriends."
In addition, Hefner said things have been getting "more serious" between him and the twins' fellow The Girls Next Door star Crystal Harris -- however a trip down the aisle apparently isn't in their future.
"I haven't had a lot of luck with marriage," Hefner told E! News. "I don't have a good track record and I don't want to screw this one up."
Karissa, Kristina and Harris moved into the Playboy Mansion as Hefner's new girlfriends prior to The Girls Next Door's sixth season -- which also featured the departure former girlfriends Holly Madison, Bridget Marquardt and Kendra Wilson.
The Girls Next Door's sixth season premiered in October and Hefner told E! News should the series get renewed for a seventh season the twins will still be part of the cast to show "how my relationships are changing romantically and how that relates to Crystal."
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