When it speaks of the Sacrament of Holy Orders and who may receive the sacrament, the Catechism of the Catholic Church does not distinguish between the order of diaconate and priesthood. It says simply, "Only a baptized man (vir) validly receives sacred ordination.'" It goes on to say, "The Lord Jesus chose men (viri) to form the college of the twelve apostles, and the apostles did the same when they chose collaborators to succeed them....The Church recognizes herself to be bound by this choice made by the Lord himself. For this reason the ordination of women is not possible" (#1577).
Some theologians argue that, cultically, only a male can adequately represent Christ in the eucharistic liturgy. I believe that the strongest argument against the ordination of women to Holy Orders is tradition and the appeal to authority.
Pope John Paul II concluded his 1994 apostolic letter to all bishops in the Catholic Church, Priestly Ordination Reserved to Men Alone (Ordinatio Sacerdotalis) by writing, "In virtue of my ministry of confirming the brethren (see Luke 22:32) I declare that the Church has no authority whatsoever to confer priestly ordination on women and that this judgment is to be definitively held by all the Church's faithful."
In other words, Pope John Paul II believes that it was determined by Christ himself that only males be ordained to the priesthood and that he, John Paul, has no power to deviate from what Jesus has determined.