Question pertaining to liberals' interpretation of fairness?

I recently read a piece in my newspaper about a teacher not allowing a child to p out invitations to his 2nd grade birthday party because he had invited all boys and this was not considered "fair." The boy cried and was very angered by this, as he has every right to be. In second grade, most little boys' friends are other boys, as they think girls are icky at that stage. I was never invited to a girl's birthday party until 7th grade. I was not emotionally scarred because of this, I, as any other child of my age, considered it perfectly normal to invite all boys to my parties, as my friends did the same, as did the girls with other girls. The definition of fair is as follows: free from bias, dishonesty, or injustice: a fair decision; a fair judge. As a child, our natural differences in gender create bias. We do not act out of bias, just what comes naturally. So to try and correct the natural bias at this stage, would be trying to ignore the inherent differences between the genders. To make someone invite a member of the opposite as a prerequisite to ping out invitations at school would be forcing "fairness," upon the kids, thus disqualifying it as being fair in the first place. What are your thoughts on this matter? Is this truly "fair?" How could it be? I am not trying to incite heated responses, just to understand this, in my opinion, very convoluted point of view.