Justin Timberlake cheated on Jessica Biel with Olivia Munn

The new Us magazine says that Justin Timberlake banged Olivia Munn for three straight days this past September, and who could blame him for that, except of course his girlfriend Jessica Biel. Who probably, instead of congratulating him, will get all whiny about it.

Timberlake, 29, and Munn, 30, first met at a Sept. 26 MySpace event. After exchanging numbers, he "started chasing her immediately," says an insider. Munn resisted his advances, telling him it was a no-go if he was still with Biel, 28. But Timberlake "has been telling people it's over with Jessica, even though "the reality is he's just doing it behind her back." Believing he had left Biel and that her relationship with Timberlake might develop into something serious, Munn took him to her hotel, the Gansevoort Park Avenue NYC, Sept. 27 and 28, where "they were openly affectionate," the insider adds. The insider tells Us the two "had amazing sex" that night.

If I ever get caught up in a lurid Hollywood sex scandal like this, I hope I have a source who says things like we had amazing sex. If they said, "Brendon mostly apologized and then cried," you can see how that wouldn't be quite as good.