There are two threads currently active on this same topic so I apologize if this is a repeat.
The argument is NOT that female altar servers directly diminish vocations. Rather, the point is that by making altar serving a co-ed oportunity, some of the oportunites that boys have to consider a vocation are lost or diminished.
For example, there was a poster on the other thread that talked about outings the altar servers went on to the seminary, dinners with the Bishop to discuss vocations, father-son activities. All of those activities are aimed at helping boys discern if priesthood is something to consider and none of those activities could be done with girls in the mix. This is something we should promote.
In some parishes, there are limited spaces available for servers. Having girls means fewer “spots” open for boys or that everyone serves infrequently. It’s just the math.
There is also the fact that, at least in some parishes, girls take the place of boys by driving them away, purposely or not.
In my own parish, we had a family with three girls who were very eager to serve often. They would come extra early to Mass, get dressed as servers and when the scheduled servers arrived (on time), they would tell the boys “too late, we got here first”. That didn’t happen too many times before the boys gave up. Father had to step in and tell the girls to stop doing this, which of course made their mother quite angy.
There is no real doubt, in my mind at least, that serving at the altar is good for girls. But it **may be **bad for boys and it may not be good for the Church as a whole - either the larger Church or the local parish. That is why the Church lets the Bishop decide for the diocese and, if the Bishop allows girls, the priest can decide for his parish.