It isn't the genitals that you are targeting, it is the pelvic bone. A shot to the pelvis is almost instantaneously disabling, but it not a reliable threat stopper.
There are not many instances in which this knowledge is useful because it is never a primary target. However, if you do find yourself in a gun battle, any hit is better than no hit. A shot to the pelvis that anchors your assailant would allow you to escape more easily.