Well, if it's not wrong, then what's the harm of having them up there?
Simple: altar girls are vocation killers.
When a boy is of age to begin serving at the altar, he usually doesn't want much to do with girls.
If his parish has altar girls, he will be far less inclined to serve. Other boys will also be so disinclined, and will not serve. Altar girls will fill the role, and the role becomes feminized.
If the traditional role of serving at the altar becomes feminized, because it has been a traditional source of vocations, the priestly vocation becomes feminized, and stigmatized in the minds of young boys and adolescents.
Only a boy with enough precocious self-confidence to brave ridicule--or one with homosexual tendenices--will serve in the role.
Hence we can see that allowing altar girls have not only contributed to the decline in priestly vocations but also to the homosexualization of the priesthood.