Here is a serious response for you Gen,
First off you are shutting down an entire side of your body from being able to defend if you go for the groin and plan on holding onto it. I 100% guarantee that one of the first reactions from a male would be to hit the one trying to squeeze his groin. In this case you would have one less arm to help defend. I could see a female getting knocked out especially if a grown male is already on top. Which leads me to my question for you: If you were squeezing and the man on top started hammering you in the face would you continue to hold on? I would assume the retractionary response would kick in and you would go into defensive, not offensive mode; this is common in all humans. The male that is getting his groin area squeezed knows it is either submit or fight and I would be willing to be 98% of the time he would want to fight. Now, in a calm state this is another story. If a female grabs onto a males genitalia without any warning she is by far in control.
Now onto my next point:
There are extremely sick people out there that actually get off on women doing genital manipulation to their genitalia. I am venturing a guess that if a male was trying to rape another female they may easily be one of these sickos. In this case you may simply turn them on more rather than deter them.
Gruhn