How should one explain to a Protestant why it was necessary for Mary to be conceived without original sin but that it does not naturally follow that her parents (Anne and Joachim) would have also had to be immaculate?
Original sin is the deprivation of sanctifying grace. In the present condition of the human race, we cannot function properly if we have been conceived with original sin. Being conceived in this way damages the person such that they are born with a fallen nature. This damage is known as the stain (Latin, macula) of original sin.
To cause Mary to be born without original sin and its stain (i.e., immaculate), God infused sanctifying grace into her soul at the moment of her conception. Mary was given the special privilege of being conceived without original sin (with sanctifying grace) and without the damage or stain such a conception causes. This was done to equip her with the graces needed to make her a fit Mother for the Redeemer and for the Church.
Mary’s preservation from original sin was accomplished in anticipation of her Son’s redemptive work. Therefore, Jesus is also Mary’s Savior. Because of what he would do on the cross, this grace was given to her early. The gracious character of this blessing is also the reason that Anne and Joachim did not need to have it: It was a grace God could give to anyone at any time. He chose to give it to Mary to make her a fitting mother for his Son.
This grace was not given to Anne and Joachim because it was not fitting for them to have the same precise graces as Christ’s own mother, who bore him in her womb. No doubt they were very blessed in many ways, but not as blessed as their daughter.