Perhaps I can offer the opinions of an altar boy.
At my parish, our priest celebrates Mass according to both the current missal and the missal of 1962 (Novus Ordo and Tridentine). For Mass according to the Novus Ordo he allows female altar servers whereas, keeping with the ancient tradition of the Roman rite, he allows only males to serve at the Tridentine Mass.
I only serve at the Tridentine Mass. As such, I have never had the opportunity to serve with a female altar server; however, most servers I know have. The general consensus among younger altar boys (10 down) is that they simply do not like serving with girls. Many dislike it to the point that they would quit serving entirely.
One point that I am surprised no one has brought up is the ideas of older altar boys who serve with altar girls. It is a simple fact that having female altar servers, almost always through no fault of the altar girls, exposes many altar boys to near occasion of sin. Once again I rely upon anecdotal evidence, but the vast majority of altar boys (though they might not admit it) are focused more on the altar girls than the Holy Sacrifice of the Mass.
Certainly these are not the only reasons I support the laudable tradition of having only boys serve at the altar, but it seems that this conversation was lacking the opinions of someone “in the trenches”, so to speak. As per usual, I was more than happy to oblige and give my two cents.