A crossover episode starring some of the funniest animated characters in TV history is drawing few laughs after Fox network's "The Simpsons" and "Family Guy" special scheduled to air Sunday has drawn criticism for featuring a rape joke. Fox has so far ignored requests from rape advocates and parent groups to drop the joke that includes the punch line "your sister's being raped."

The much-anticipated crossover episode has been heavily promoted by Fox. It will feature two of America's favorite animated families -- the Simpson and the Griffins. Both shows are known for off-color humor, especially "Family Guy."

During the episode, Bart Simpson teaches Stewie Griffin how to make a prank call. The boys dial the owner of Moe's Tavern and ask for a patron named Keybum, first name Lee. When Moe asks if there's a "leaky bum" in the room, the boys and the bar patrons laugh. Stewie then makes his own prank call. "Hello, Moe?" he says. "Your sister's being raped."

Tim Winter, president of the advocacy group Parents Television Council, said he found the joke particularly offensive amid a national debate over how women are treated after a series of high-profile sexual assaults on college campuses and the NFL's domestic abuse problem. "I was blown out of my shoes when I saw the scene with the rape joke in it," Winter told the Associated Press. "It really troubled me."

Katherine Hull Fliflet, spokeswoman for the Washington-based Rape, Abuse and Incest National Network, said the joke was educational. "I think the show is making it clear that rape is not funny by how they are positioning the joke," Fliflet said. "It's my hope that would be the viewers' take-away."

In the U.S., a woman is sexually assaulted every two minutes. At least 44 percent of all rape victims are under the age of 18 and 60 percent of all sexual assaults aren't reported to police, according to the organization.

Family Guy creator Seth MacFarlane predicted the joke would cause a stir in a recent interview with Entertainment Weekly. "But in context," he told the magazine, "it's pretty funny."

Watch a trailer for "The Simpsons" and "Family Guy" episode featuring the rape joke below.