It seems that Ma and Pa had married under none too happy circumstances, and their married life has been a bit rocky since then. Apparently Ma's family decided that Pa was taking too long to pop the question, so a modest amount of external encouragement was applied.
After forty years of marriage Pa went to the local judge to ask for an annulment. The whole town gasped with amazement. A date for the hearing was set, however, and when the time came the judge demanded to know the grounds on which Pa based his demand for an annulment.
"It's like this, Judge," says Pa,
"I've just learned that Ma's Pa never had a license to carry a gun ..."