Sienna Miller actually had sex with Hayden Christensen while filming love scenes for 'Factory Girl', a film insider has claimed.The explicit scenes between the pair - who were romantically linked during the movie - are so realistic because they were allegedly actually having sex. An on-set insider said: "It's not simulated, they're really doing it."Sienna has slammed rumors they were having real sex during the scenes. Standing with her parents after Tuesday's screening at New York's Tribeca Grand hotel, she said: "Oh darling that is so horrible. You obviously don't know anything about making movies."Sienna's representative, Leslie Sloane Zelnick, said the scenes were convincing because of Sienna's acting talents. She told the New York Post newspaper: "It's not true at all. When you do a love scene, there are five or six people on the set at a minimum. They weren't having real sex. She's just a great actress."Director George Hickenlooper refused to comment on the scenes, insisting:"You'll have to ask Sienna about that. We tried to portray it tastefully."Sienna and Hayden grew close during the filming. It was an emotional experience for us all."Sienna, 25 - who plays 60s socialite Edie Sedgwick in the movie - recentlyadmitted she was worried about her parents seeing the racy scenes. She said: "We wanted to make it realistic and I watched it thinking, 'Oh my God, my dad's going to see it!'"But we didn't want to hold back because it is a real and gritty film. It wouldn't fit in the film if we had an unrealistic sex scene."Sienna and Hayden, also 25, reportedly dated for a month following Sienna's split from Jude Law. A friend of Hayden's revealed to the New York Daily News newspaper: "They spent about a month hanging out. But then she decided she didn't want a relationship. Hayden was devastated. He really fell for her."

Sexy Hunk Hayden Christensen