[Flying Circus]

Whether or not there is a girl standing next to a 10 - 14 year old boy (the age of altar servers) during the mass is not going to affect a young man's decision to enter the priesthood when he is 18 - 20.

The girl's presence at the alter probably means there is one less boy there. If she is not out-right displacing the boy, having her fill the position means there is less ipetus to find a boy to fill the position. As it stands at many churchs today, all of the alter servers are girls and there are no boys. Being an alter server increases the chances of a boy later seeking the priesthood, therefore we should seek to have as many positions filled with boys as possible.

[A. Pole]

The girl's presence at the altar probably means there is one less boy there. [...] As it stands at many churches today, all of the altar servers are girls and there are no boys.

There is one important psychological characteristic of boys - they instinctively do not want to appear or be "girly". So girls can push boys out, at least those boys who should be priests. :)