Obviously I seem quite biased, but I doubt there is any equivalent pain on a woman to a man getting nut-kicked. Sure a good poke to the boob is quite painful according to my wife, but it takes an very small amount of pressure to a testicle to cause discomfort. Anything beyond very slight pressure is gut wrenching and being kicked there will incapacitate you for an extended period of time as the pain reaches up through your stomach almost pulling at your lungs.

There's simply nothing else like it. I'm sure there's a solid evolutionary reason for this considering from the evolutionary perspective nothing is more important than passing on those genes.