Why can fraternal twins de different sexes while identical twins cannot?

To keep it simple and not get too technical, fraternal twins are from two eggs and two sperm that independently combine to produce a baby. Identical twins come from a single egg and sperm that divide to create two separate children. Gender is controlled by the males sperm and whether it contains the male genetic X chromosome. Since identical twins have the same sperm their genetic makeup is exactly the same (same egg and sperm used in development). Fraternal twins have two separate sperm and therefore it is possible one has the X chromosome and other does not.