As revealed in the new Us Weekly, complete with exclusive photos, the actress, 22, was caught in several steamy makeout sessions with Rupert Sanders, her 41-year-old Snow White and the Hunstman director, who is married with two kids.
No word yet on exactly how her Twilight costar and real-life love Pattinson, 26, has reacted to the betrayal -- but an interview with the April issue of Italian Vanity Fair offers heartbreaking clues.
When the magazine asked the British hunk whether he could ever date a married woman, Pattinson replied, "Life is not black and white. There are married couples that never see each other. Is that marriage?"
He added tellingly: "But there's a thing I've never got: That is, why do people cheat?"
"I can understand the impulse," Pattinson continued. "But not how you can keep two relationships going at the same time for long."
Indeed, before Stewart's scandal hit, pals told Us that Pattinson was preparing to propose to his Stewart.
"I'm not the casual-affair kind of guy," he told Vanity Fair. "If I choose to be with someone, it's because I really want it. When I have a relationship, I'm 100 percent into it."
Pick up the new Us Weekly, on stands Friday, for all the exclusive details on Stewart's shocking fling.
This article originally appeared on Usmagazine.com: Robert Pattinson: I've Never Understood Why People Cheat