In my karate class, the girls used to steal our protective cups, then they would cup check us.  That is when you try to kick a guys balls, but hit his cup, to make sure he is wearing one.  It never made sense to me, but the female instructor said it was an important part of class.  She felt cup checking would save a guy from a real hard and traumatic kick to the testicles during sparring.  I thought it was weird to have my balls kicked, to make sure I didn't get them kicked really hard.  I also have a problem.  I get hard after receiving a kick to the balls by a girl.  So, when this little 13 yr old girl kicked my balls from behind, my cock got hard, even though I was wearing a cup.  Well, my boner was not being contained by my cup as I held and rubbed my balls.  The female instructor came over, saw my situation and started laughing.  The sneaky little girl who kicked my balls told me I should go jerk off to make sure they still worked.  My instructor helped me into the bathroom.  She asked how my balls were and said I should get out of my cup asap, then left me alone in the bathroom.  I started to whack off w/o out knowing the girls from the class all had their ears against the door and listened to me pleasuring myself. After I came, I heard them all laugh and tell my instructor that my balls still worked.