It's a pretty common opinion among my generation of priests that the presence of female altar servers makes boys less desirous of serving at the altar, and therefore less likely to hear the call to priesthood. I strongly disagree. I'm tired of the statistics being twisted (the ones that accurately show that most new priests were once altar servers) to suit the polemic against altar girls.
It's not girl altar servers that push boys (or anyone) away from altar serving. Rather, it's sub-par altar server training. It's the failure to give children actual responsibilities. It's lazy and lackluster liturgy. All of those things have been pretty much concurrent with the presence of female altar servers. To blame the middle/high school age altar server exodus on mere presence of girls is to completely ignore the real issues.
I was an altar server. One of my fellow priests was also an altar server with me, same age, same parish. There were a ton of female altar servers too. Neither of us were driven away from serving nor from the priesthood. Why? We loved serving. We were taught well, and we had responsibilities. We enjoyed being good at what we did, and we didn't want to let anyone down. That's really all it took to keep our interest. The gender of the participants had nothing to do with it.
Furthermore, the presence of female servers in no way hindered the parochial vicar (who was in charge of the servers) from encouraging priestly vocations all the time. We heard about it on a regular basis, and in fact, that's where I got the idea.
If you are half-hearted in your training of altar servers, you'll get half-hearted servers that think what they're doing is lame. And that's how you lose altar servers. Blaming it on girls is a red herring that needs to go away.