The sensor sometimes goes off, sometimes it doesn't. If it does, the security just frisk you (over your clothes) and are happy enough I'm not hiding a gun.
In order to trigger the metal detectors, there would have to be a fairly large (and probably highly uncomfortable) piece of metal inserted into the piercing. I can usually leave my belt (with a fairly large metal buckle) and watch on without triggering the sensors. The thought of carrying an even larger amount of metal in an 'intimate' place brings tears to my eyes! ;-)
Can someone please explain why the sensors go off 9 times out of ten when I go through airport security with nothing whatsoever metal on my person or my clothing. I am fed up with being frisked!