There’s something especially satisfying about a girl kicking a guy in the balls.
See, if a guy gets kicked in say, his shin, he feels pain, but a girl is capable of feeling the same thing.
Whereas if he gets kicked in the nuts, it’s a uniquely male suffering, she’ll never have to experience it.
I think these girls laugh not only because causing pain is fun, but also the knowledge that they don’t ever have to feel it. It makes them feel superior.