I know this is an apoc thread, but I have always found this question fascinating.
If balls were such a weak point, they would have migrated back inside at some point. Balls aren't THAT hard to protect. They require a the assaulter to be close, and the assaultee must be not prepared to protect them.
Not to mention, the entire animal kingdom also has balls. Lions don't go after the balls of zebras, because they're not that much of a weak point.
Sure a kick in the balls hurts, but only if you're not prepared and the person is right up in your face. It's not a surefire incapacitation like a blunt object to the head.
There are obviously many examples of women fighting in history - the women of the steppe tribes often couldn't get married without having killed an enemy in battle. They also used bows that were considered to require inhuman strength to fire. It's just that it's very clear, that the physical strength and stature of a man dominates that of a woman. Weapons can even the odds, but balls don't.