I've found that, when I'm aroused at least, my balls are pretty resistant to pain. My girlfriend can slap, flick, or even punch them, and I feel at most a mild discomfort. I'm sure a full-strength kick would still be agonizing, but we've never gone that far. Sometimes she slaps me pretty hard though...she winds back her arm and really lets them have it. This sends her into giggling fits, of course. But the first time she did that (at my behest), I expected it to put me on the floor, and instead I didn't even feel the urge to double over...weird.
Has anyone else experienced this?