It would seem that the restriction of Ordination to men is actually not a deal breaker for most Catholic women; although few rank-and-file Catholics actually agree with the Church that she has no authority to ordain women, few people leave the Church for that reason alone. Some women become "sisters in waiting," working like secret op agents to "change the Church from the inside." But generally, this doctrinal chestnut is neither agreed with nor very vigorously challenged, except within organizations founded for just that purpose.
Still the catechist has a prickly task in teaching the issue. I do not agree with some catechists who argue that the issue should not be raised by the teacher. If the Church really believes what she teaches, her pastors and catechists should not be embarrassed about it, but should instead try to understand things more and to help others do the same. Incidentally, with umpteen recent magisterial documents (and several books) about women, and few to none about men, one wonders whether there is not a guilt complex operating within the Vatican as well.
However, what those documents on women reveal is that the doctrine on the Ordination of men alone can actually be an occasion to open up deeper issues about the Catholic faith--the organic structure of the Body of Christ, the richness and height of the stature of the feminine within revelation (including the Marian component of the Christian life necessary for the salvation of all, including men, and exemplified equally by Christ), the priestly office exercised by all Christian women and men by virtue of their baptism, etc. It can also open more tangential subjects, like the relation between revelation and culture, the nature of equality, the distinctive role of the ordained clerical state (and it's non-priviledged status before the judgement seat), and the Biblical theme of the "scandal of election". It can be an occasion for the catechist to demonstrate unusual candor in coming to terms with real misogyny in Christian history and Catholic tradition--but also to unveil rarely mentioned occasions in Catholic tradition of bold egalitarianism.
My point is that the doctrine of the restriction of Ordination to men can be used as a doorway to discuss virtually endless issues--that is one of the magnificent things about theology: everything is connected to everything else. That is not to say that I suggest making that doctrine a cental component of catechesis (that would be a little wierd), but it does open up some possibilities.
But sometimes, going so much deeper is not necessary. Sometimes, the simplest answer is the best one. Sayeth the Catechism:
"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 in their ministry. The college of bishops, with whom the priests are united in the priesthood, makes the college of the twelve an ever-present and ever-active reality until Christ's return. 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." CCC 1577
Although, nowadays, it is unlikely that young people will be satisfied with this explanation, there are some who will be. Some will see Christ's example as absolutely normative and thus will understand the Church's doctrine to be strictly a matter of humble obedience; not a smokescreen to justify an oppressive regime. It is true that boys and men are less likely to question it than women; but I would suggest that this is not because they subliminally or secretly feel gratified by a "favorable" inequality. Studies in Belgium have shown that Catholic boys typically respond more immediately to structures of authority--for example, prior to Vatican II, it was illegal for anyone but the priest to touch the consecrated host with his hands. If an old priest dropped a host on the floor, boys would assist the priest so that he could pick up the host. The girls, more often, just picked it up themselves. (That was the account of my psychology of religion professor--I don't have a citation).
It might be that students will have been told that the reason women cannot be priests is because Christianity teaches that women are more sinful, because Eve was the first to fall. In this case, it might be helpful to reference the Catholic Answers collection of quotes from the Church Fathers, which do not use Eve or the Fall to justify the ordination of men alone--the closest is the third passage from the Apostolic Constitutions, which quotes Gen 3:16, "...he shall be your master." But immensely more common among the Fathers is the teaching that the ordination of men is rooted in the example of Christ: the Didascalia, Epiphanius of Salamis, and the first passage of the Apostolic Constitutions.
It is important, if the Fathers are used, that the students understand that what an individual 'Church Father' says is not as important as what their overwhelming consensus is. Also, the Fathers' more shocking statements, if they are shown to the students, should be placed in the context of a Church struggling to maintain the purity of its doctrine in the face of opposition from Gnostics and Greeks. It might be helpful to cite Eusebius's Church History on the persecutions and the gnostics.
I would grant that a certain degree of chauvanism was almost universal in the first centuries of the Church. But I would also point out that Christian women did enjoy a revolutionary degree of human rights relative to non-Christian women. Ironically, the presence of priestesses in gnostic and Greek religions did not translate into higher standards or quality of living. Christianity was something an innovator in its requirements of chaste fidelity by men, and its teaching that wives had substantial rights within marriage, to say nothing of the dignity held by widows, virgins, deaconesses, and other lay women. Women martyrs and confessors were venerated and their prayers considered just as powerful as those of men.
But the most important lesson is that the Church has always considered the Ordination of men to be the inviolable pattern set by Christ and the apostles. In other words, that was not an sweetened excuse cooked up by Vatican II to cover up some kind of "Original Misogyny"--it is, and in a certain sense should only be, rooted only in the earnest obedience offered by Christian disciples from the very beginning. In other words, "Hey, it's nothing personal."
From here, the conversation will likely take a different path. Most commonly, someone will suggest that Christ's selection of men alone for the Twelve was not intended to set an ecclesial norm, but was only done out of convenience, in view of the prevailing chauvanism of his day. We should not extrapolate from Christ's decision a binding norm--as the argument goes--because if priests have to be men, why should they not also be bearded, ethnically Jewish men? (A variation on this, involving the priest's resemblance to Christ, also satirically asks, "why not require that they also be carpenters?")
Normally, the conversion gets caught in a stalemate at this point; the questioner will suggest that it would have been impossible for Jesus, along pragmatic lines, to have women among the Twelve. The teacher will respond (with the Magisterium) that Jesus did several 'impossible' things with respect to women; yet he did not elevate any of them to the Twelve, which thus indicates that it was his conscious will. The questioner will then, and correctly, point out that Jesus was not consistently counter-cultural. Indeed, sometimes practical circumstances did prevent Jesus from carrying out his conscious will. One example is the cured leper in Mark: "He spread the report abroad so that it was impossible for Jesus to enter a town openly. He remained outside in deserted places, and people kept coming to him from everywhere" (Mark 1:45).
The whole thing winds up in an interminable back-and-forth about how much, or how little of Jesus', messianic will was behind his decision not to count among the Twelve his mother Mary, Mary Magdalene, or his other female disciples. It is based on two supposed premises, summarized nicely by John Wijngaards, an author of an online book arguing for women priests,
"If in selecting only men for the apostolic team Jesus was guided by the general practice of his own times, we have no reason to presume his objection against the ministry of women in changed circumstances. If however Jesus broke with the social myth of male predominance and yet refused to admit women to the apostolic team, we have a clear indication that he was setting a permanent norm."
In fact, neither of these premises is precisely true. The second premise makes it seems as though Church's forms are patterned off of a one-to-one imitation of the earthly ministry of Jesus Christ. It forgets that it is not the recorded life of Jesus alone, nor Scripture alone that determines even the core dogmas of the Catholic faith. It is the entire received Tradition which indicates more fully how Jesus was received and understood by his first disciples. Jesus' election of men, even if it stood out amid a rebellious ministry, would have no binding force on the Church if Peter, Thomas, John, and Paul began 'laying hands on' women. This would not have been so strange, given the strongly Greek context of the early Church. She was forced, after all, to relinquish other laws of her Jewish heritage in order to adapt. Ordaining women would have illustrated that the apostles did not feel compelled to follow Jesus' pattern. But they did not alter the male constitution of the clergy.
But Wijngaards' first premise is, perhaps, even more off the mark. Jesus' being "guided by the general practice of his own times" had no bearing, one way or the other, on what eventually became normative for his Church. This does not mean his words and actions were not normative. But whether Jesus was guided by his culture or rebelled against it had absolutely no bearing on the developing Tradition. Not incidentally, the deepest elements of Catholic liturgy are Jewish in origin, and countless other elements of the faith--even permanent ones--come from historically conditioned cultural forms.
This is not yet a positive argument for the restriction of Ordination to men alone; but it does expose a certain popular fallacy attached to the notion of "inculturation". Catholicism is not a disembodied "ghost religion"; it has a body, flesh and bones, a birthday, a mother and father, living children, and even its favorite music, language, and architecture. It does not lose its flesh and bones--its historically conditioned forms and expressions--when it is transferred from one context to another. Yet neither does Catholic culture steamroll other cultures in some kind of imperialistic kulturkampf.
In his book "Truth and Tolerance," then-Cardinal Ratzinger put it this way:
"A first point we should note is that faith itself is cultural. It does not exist in a naked state, as sheer religion. Simply by telling a man who he is and how he should go about being human, faith is creating culture and is culture... The people of God, as a cultural agent, differs from the classic cultural agents, which are defined by the boundaries of a communal life as a tribe, as a nation, or otherwise, in that it subsists within various different cultural entities, which for their part do not thereby cease, even for the individual Christian, to be the primary and immediate agent of his culture... man now lives within two cultural entities: in his historical cultura and in the new one of faith, which meet and mingle in him. [...] It is the tension itself that is productive, renewing the faith and healing the culture" (Ratzinger, Truth and Tolerance, 67-70).
So the argument, that Jesus's election of men alone to the Twelve was guided by Jewish culture, does not have the force it intends. The idea that religion can be separated out from culture into a kind of pure, skeletal form is strictly modern, and it would have the effect of reducing Catholicism into an "empty collection of ideas" (Ratzinger, 70).