Why is PGD (pre-implantation genetic diagnosis) a bad thing to use for gender selection of your child?

Many nations ban the practice, and many doctors here will not screen for it when patience desire it for gender selection. Why is that? If a family wants to balance out their children at least they are creating or providing the way to life, not ending it, they should be able to have some direction over the process when science allows. If a couple has two of the same gender and want to have one of the opposite gender, etc to balance things out, why shouldn’t they be able to use PGD (pre-implantation genetic diagnosis) to achieve that? It cannot be as harmful as using science to stop a baby altogether.