Shelby Stivale / Us Weekly: Lena Dunham Reveals She Cheated on Jack Antonoff Before Split: Biggest Bombshells From New Book - Lena Dunham discussed her relationship with ex Jack Antonoff at length in her newly released memoir, Famesick. - "The best medicine was Jack. Here was this boy - no, at 28, he was a man," Dunham, 39, wrote.