Its not about becoming boys – I think boys would not want to be girls and girls would not want to be boys if they really understood what it meant to each of them.
It about making a statement to girls that their coming of age isn’t significant when you throw a big bash for the boys that is not connected to the mitzvah. Their coming of age is just as significant as boys, but they have different obligations.
If you are going to throw a large party for a boy, especially when its not relating to the mitzvah you ARE sending the message to her that she is less significant.