A traveling salesman is driving down a country road when his car breaks down. He hoofs it for a while and after several miles comes upon a farmhouse and knocks on the door. An old farmer answers. The salesman asks if he can use the farmer's phone to call AAA; the farmer says sure. The salesman calls, then hangs up. He tells the old farmer AAA won't be there until the next morning and asks if he can crash at the old farmer's place for the night.
The old farmer, leery of strangers in his home, says no, but that he can sleep in the loft of his barn. The salesman figures that's better than nothing and agrees. The old farmer takes him to the barn, where the salesman sees a wall with a line of three holes, each at crotch-level. He asks the old farmer the deal. The old farmer tells the man that whatever he does, under no circumstances should he stick his dick in any of the holes. The salesman thinks to himself, yeesh, that's a weird thing to say. He shrugs it off and the old farmer shows him to the loft, then goes back to the house.
That night the salesman's laying in the loft, staring at the barn ceiling, and he can't get what the farmer said out of his head. "Why would he say that? The thought of sticking my dick in those holes didn't even cross my mind. But now that he's said it...god, I just can't stop thinking about it."
Finally his curiosity gets the best of him. He climbs down from the loft and goes to the wall with the three holes. He stares at them for a minute, mind racing. What could possibly be in those holes that I shouldn't stick my dick in them? Finally he says screw it and drops his pants.
He sticks his dick in the first hole. Wow. Whatever's in there, it feels amazing. But the second hole beckons. So he shoves it into the second hole. Holy shit! That feels even better! The third hole is probably going to be even better still! So he moves to the third hole and sticks it in.
Inside the house, the old farmer's startled awake by a piercing scream from inside the barn. He throws on his house coat and races out to the barn, where he finds the salesman curled up fetal on the floor, clutching his genitals and writhing around in the most unimaginable agony you can imagine. "Jesus Christ," he says, "what the hell was in those holes?!"
The old farmer says, "Well, the first hole was my wife, and she's got the clap. The second hole was my daughter, and she's got the gonorrhea. And the third hole was the milking machine, and it don't stop 'til it gets a quart."