Sorry no pie-charts or cute graphics, but here's some medical information that has some numbers:

http://author.emedicine.com/MED/topic2859.htm

In this text the author says "The tunica albuginea is the layer that is violated during a testicular rupture. Approximately 50 kg of force are required to rupture the testicle..."

Whether it's 50kg (110 lbs) like this author says or 60 lbs like Sara says, that's still less than most women weigh, so if she stands with her weight completely on your nut on a hard surface, you're either unique or eunuch. (punny -eh?)

-Tamakeri