The 29-year-old actor made the revelation while being interviewed by Andy Cohen on "Watch What Happens: Live". Eastwood recalled an event in which Kutcher spent his sixth wedding anniversary in San Diego without his wife in September 2011.
Instead of celebrating their wedding anniversary, Kutcher allegedly cheated on Moore with local resident Sara Leal and her friend, reported Ace Showbiz.
Kutcher's alleged cheating scandal was brought up in the conversation when Cohen asked Jon Cryer, who also joined the interview, about Kutcher.
"If it makes you feel any better he did sleep with one of my ex-girlfriends," Cryer said.
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Then Eastwood interrupted, "But she was my current girlfriend at the time so..."
Eastwood insisted that it was true, confirming that she was "the girl in San Diego." He added, "Yeah, it was her and her friends."
"I think it was the catalyst that broke him and Demi up," the actor claimed.