A guy goes to see a doctor because he's ...well, a little too well endowed, shall we say. In fact, it's 25 inches long. He can't get any women to have sex with him. The doctor says there's nothing he can do medically, but sends him to see a witch that he thinks might be able to help. The witch takes a look at the problem and tells him, "Go to a particular pond, deep in the forest, and talk to a frog that lives there. Ask the frog to marry you and each time the frog says 'no,' you'll be five inches shorter."
The guy decides it's worth a try and dashes into the forest, as anyone in this sort of joke would, finds the pond, and sees the frog on the other side, sitting on a log. "Will you marry me?" he calls to the frog.
The frog looks at him, disinterested at best, and calls back, "No."
The Guy looks down and sure enough, he's five inches shorter. "Hey, this is great," he thinks to himself, "Let's try it again."
"Will you marry me?" he asks the frog.
The frog rolls his eyes, and shouts back again, "No!"
Twitch! The guy's down to 15 inches. "Well, it's still a bit excessive," he thinks. "Down another five would be perfect." So he calls across again, "Will you marry me?"
The frog yells back, "Look! How many times do I have to tell you? NO! NO! NO!"