This is not true on just so many levels. Lots of people have mothers not worthy of the name. I don't know exactly what fatherhood being a theory and motherhood being a fact means but I struggle to find any that isn't offensive to fathers.
I take his quote to mean that motherhood is more difficult than fatherhood, especially back in the time he was active. The quote should, I feel, not be observed as a factual statement, but rather in the vein of his politically based humor. Largely, it was a simpler time.