One day, Farmer Brown went behind his barn and found to his dismay that his son, Jeb, was jerking off. He vowed to his son that he was going to help him find a wife, so he would not have to be doing this.
Sure enough, the father was able to find a suitable bride just right down the road and the couple was married shortly thereafter. Six weeks later the farmer was again going behind the barn and caught his son vigorously jerking off. The farmer went berzerk.

"Why are you still doing this, why aren't you with your wife?"

"Aw Paw," said the son, "Her little old arm gets so tired."