As is well known, female alter servers were permitted in 1992, after a long campaign of disobedience towards the Church's norms. It was permitted with considerable reluctance, and clearly the motivation was to avoid the scandal of open warfare on the subject: Pope John Paul II was far from convinced that it was actually a good idea. A 1994 ruling from the Congregation for Divine Worship hedges the matter about with caveats and reservations, which are still in force. Having conceded the possibility of altar girls, it goes on:
2) The Holy See respects the decision adopted by certain Bishops for specific local reasons on the basis of the provisions of Canon 230 #2. At the same time, however, the Holy See wishes to recall that it will always be very appropriate to follow the noble tradition of having boys serve at the altar. As is well known, this has led to a reassuring development of priestly vocations. Thus the obligation to support such groups of altar boys will always continue.
3) If in some diocese, on the basis of Canon 230 #2, the Bishop permits that, for particular reasons, women may also serve at the altar, this decision must be clearly explained to the faithful, in the light of the above-mentioned norm. It shall also be made clear that the norm is already being widely applied, by the fact that women frequently serve as lectors in the Liturgy and can also be called upon to distribute Holy Communion as Extraordinary Ministers of the Eucharist and to carry out other functions, according to the provisions of the same Canon 230 #3.
4) It must also be clearly understood that the liturgical services mentioned above are carried out by lay people ex temporanea deputatione, according to the judgment of the Bishop, without lay people, be they men or women, having any right to exercise them.The rule against altar girls is not some forgotten rubric of the Middle Ages: it was embedded in the General Instruction of the Roman Missal when the new Missal was published in 1970, and in the new Code of Canon Law of 1983 (though certain ambiguities arising from the notion of lay people substituting for instituted acolytes led eventually to the 1992 ruling). The restrictive nature of the 1994 document was reiterated by the Congregation for Divine Worship in 2001. Indeed, as well as reiterating that it was the bishop's right alone to determine whether to permit altar girls, regardless of what neighbouring bishops had decided, the importance of male altar service for the promotion of vocations, and the fact that no right to serve is established, it emphasised that priests could never be forced to use them:
In accord with the above cited instructions of the Holy See such an authorization may not, in any way, exclude men or, in particular, boys from service at the altar, nor require that priests of the diocese would make use of female altar servers, since "it will always be very appropriate to follow the noble tradition of having boys serve at the altar".
In other words, if any bishop, or any priest, determined not to use altar girls, this would be in conformity not only with the tradition of the Church but would have the support of the Church's highest authorities, who regard such a decision as a priori 'very appropriate'.
These are legislative texts of course and not theological ones so the reasons counting against altar girls are referred to in a summary way: namely, the 'noble tradition' of the Church, and the promotion of vocations. Both are correct and of great importance, but one may still ask: why does the Church have this tradition, and why is this important for vocations? I intend to address this is two separate posts.