I thought the "below the belt" rule was made for boxing, martial arts exhibitions, and other sporting fights. In those cases, yeah, that rule should be followed.
But anyone who says things like, "I only fight honorably," or "I believe in a fair fight," has never been in a real fight. I mean, if it's just some drunk idiot messing with you and would probably fall over if he took a swing at you, no, you wouldn't take that guy apart. If you feel threatened, though, you do whatever it takes to kill him or get away.
That said, I wouldn't go for the groin or advise anyone to do it except as a last resort because that isn't the best way to incapacitate someone. What you should do is carry a knife or some kind of weapon with you at all times, and I don't mean a swiss army knife. I mean a knife. Boker Great Knight is my favorite. I also carry a pair of Delta Darts with me at all times. If you're up against someone significantly larger than you are, there's no shame in pulling a weapon.