I understand the need for Jesus to take on a sinless humanity, but a Protestant will object that if God can give Mary a sinless humanity (that is to say a humanity free of original sin) via the work of the Holy Spirit in anticipation of her role in Salvation History, then God could have, likewise, given Christ a sinless humanity in the same manner as well without the necessity of His mother being free of original sin.
ell, it is more complicated than that.  You see, Mary, St. John the Baptist and St. Joseph all have been suggested as being regenerated in preparation for the coming of Christ.  This entailed them being either preserved from original sin or cleansed of it at some point in time.  In essence they were redeemed and can still call Christ their savior as a result of this.
But Jesus was God and it would not be possible for him to assume a human nature that was not already perfect.  If he received a redeemed nature himself either by preventing the fall of his nature during conception or by cleansing it afterwards, then he could not actually be the redeemer of all mankind.  He would himself need a redeemer.
By Mary possessing a perfect human nature from the moment of her conception in anticipation of the merits of Christ, Jesus received his human nature directly from her already in a pristine state.  The pristine nature that he received from Mary did not need to be redeemed.
Mark Bonocore
July 20, 2004