Pulling his Pud
A farmer finds his son behind the barn pulling his pud, and the old man exclaims, "Son, if you are old enough to do that, then you are old enough to get married."
The next day the farmer takes his son across the hollow and arranges a wedding between his boy and the neighbor's daughter. The two are soon married and move in with the groom's father.
The next morning, the farmer goes behind the barn again and discovers his son flailing away, just as before. "Son, I got you married so you wouldn't have to do that!"
"Ah, pa, she ain't got no grip at all..."