The groin shot is not intended to be a fight stopper. It is only meant to disrupt the attackers thought process, enabling you to continue sending overwhelming counter attacks. Like Kogas said, some people have little to no reaction to a groin strike. So male or female it doesn't matter, send the groin strike but continue to follow through with other combatives. Never assume that any one tactic will work on everyone, every time. Remember Krav is about overwhelming your attacker and not giving his brain a chance to catch up to speed with your defenses. If you pause to see what you've done to them, you've given them a chance to process the situation and fight back.
If your in a "Fair Fight", your tactics suck.