"Beautiful woman with car trouble in the country" joke

A cute, young blonde is driving through the country when her car breaks down. The only house within miles is a big, old farmhouse. She goes to the door and asks the farmer if she can use the phone. The auto club tells her that they can't come out to fix her car until the next day. She tells the farmer this, and he agrees to let her stay the night as long as she keeps away from his sons, Billy and Zeb, who are innocent in the ways of the world.
Later that night, she's getting all hot and bothered and she decides to visit the two strapping teenage farm boys down the hall. She sneaks into their room and offers to teach them about the facts of life.
"Huh?" is their response.
"Don't worry," she says, "you'll like this. But I'm not getting pregnant,
so I'll put these condoms on you."
She puts condoms on the two boys and says, "There, now I won't get pregnant."
They have a wild night. The next day, the auto club fixes her car and she leaves.
About forty years later, Zeb and Billy are sitting on the front porch of the farmhouse, reminiscing about the old days, when Zeb says, "Hey Billy, you remember that girl that stopped by one night about forty years ago?"
"Yup," says Billy, "we sure had a great time that night!"
"Do you care if she ever got pregnant?" Zeb asks.
"Nope, never did," Billy replies.
"Dammit, me neither," Zeb says, "I'm taking this thing off!"