Having the girls there discourages the boys. The boys eventually think it is a ‘chick’ thing and won’t do it. But when they see other young men up there, they want to emulate them. It becomes a manly thing to do and they are attracted to it.
The same thing seems to be happening with politics.
You are right. Unless the boys have been feminized or forced, they, at that age, do not want to be a part of a group that eventually become predominantly female. The same thing is true of what used to be patrol boys as school crossing guards, etc.
Altar boys were the chief source of vocations to the priesthood. They saw not only the liturgy but many other aspects of the life of the priest as they "helped out" around the parish. Until the gays took over and changed everything, many heterosexual priests left, and women began to run things, there were strong vocations. Now look at the mess. Those in seminary work report some increase in numbers (job market bad) but the quality is not the same.