It is common to hear on the matter of priestesses "the Church doesn't have the authority to ordain women." Now in fact, I can't blame people for using this expression since that's the wording found in the English translation of Ordinatio Sacerdotalis. In fact, the Latin should have been translated "the Church does not have the power ..." The Pope used "facultatem" not "auctoritatem."
To say the Church does not have the "authority" would really mean in common speech that she does not have the "go ahead" or "the nod of approval" as it were from Our Lord.
To say that she does not have the power, however, means that there is a radical incapacity on the part of the Church to ordain women. It's not even a possibility.
A good example of the Church not merely having the "authority" would be the stipulation of monogamy. Marriage between a man and more than one women isn't strictly against the natural law in that there are circumstances where, under natural law, it could be allowed. Say a comet hit the earth and a man several women were left alive. However Christ has decreed the sacrament of marriage is to be only between one man and one woman as that is God's orginal plan in the beginning.
Now with priestesses, not even God himself could permit it without first changing the entire structure of the Catholic religion (in which case it wouldn't be the Catholic religion anymore would it?). Because the priest acts in the person of Christ the Spouse of the Church then it would be no more possible for God to permit this than he could approve marriage between two men or two women. In other words the idea of priestesses is nothing short of an intrinsic evil in and of itself.