Hey NewYorkSexMachine, a protective cup reduces the chance of a direct hit, but if it's not quite secure or if it takes the wrong angle of kick, then it's really bad. Especially the metal ones used in Muay Thai.
But yes I agree, a full-force shin kick with some body weight behind it would be 'game over'. I am pretty careful how I start off, test first with some light kicks to the body and then work up to the point where I can say "yes, just like that". Of course that's a bit contrived and takes the fun away, but after the first meet it goes better.
After a lot of meets like this, only one went a bit too far. D*ck and balls bruised for 10 days, but I must admit it was the best thing ever.
The best guys for kicking have done some kung fu, a karate style like shotokan, or tae kwondo. Those guys know how to "snap" a kick really fast so it hurts like hell, but with no weight behind it so there's no need for a trip to the emergency room.
But for me, a really nice uppercut to the balls does the trick. Ouch. :D