&gt;“I’ll do it again harder if you deserve it”

Imagine being aggressive with someone at your own discretion. Disgusting. 

Also, you should consider that sustaining repeated blows to your testicles, escalating with intensity each time may cost you a trip to the hospital and possible long lasting injuries. The idea that she could rupture your testicles if she hits you hard enough isn't too far fetched either.

It's time to set some boundaries. If a GF of mine ever purposely hurt me like that and laughed it off I'd dump her on the spot. But that's just me, someone with a high intolerance for violence in relationships.