› firstname.lastname@example.org 
Some years ago I managed to stop a boy from going too far by grabbing hold of his testicles and squeezing them hard. It was totally justified, and he was disabled by the pain, allowing me to get away from him. The thing is, I squeezed that poor boy's balls much harder, and for far longer, than was necessary purely for self-defense. I now realize that I actually enjoyed inflicting that pain on him! It gave me a fantastic feeling of empowerment, hearing a big, muscular boy begging me to stop hurting him. It has since become a sort of an obsession with me, and I take every opportunity to humiliate my boyfriends by grabbing or striking their balls. Of course, I never go far enough to cause any real damage, but I'm afraid that my obsession may be causing these boys some psychological harm and that I have to find a way to stop this habit.
Are these feelings really that unusual in girls?
I'm glad you do realize it's going to have to stop, because despite a pronounced bent toward the "whatever" in the judgmentalism department, I have to admit that your enthusiasm alarms me a little. I prefer my sadists a bit less avid, I guess.
These feelings are quite unusual in both girls and non-girls. But since this planet is nothing if not generously populated, that still makes for a fairly large number of people who enjoy acting out their aggressive fantasies on the testicles of any males within reach. Furthermore, ball-kicking/punching imagery and "oww, my balls"-type humor are so prevalent now that even those aggressors for whom it is not a particular turn-on cannot help but at least consider it on occasion.
I have registered my dismay over testicular-injury humor before, and pointed out that a simple gender switcheroo instantly renders any such "joke" not only not-funny but actively appalling, and have pled for an end to "kick him in the balls, har har har" as a mode of discourse. To little avail, obviously.
For you, the question of prevalence may be interesting but is ultimately irrelevant: no matter how many people do it (not that many, and many of those for money), you need to quit it for your own psychological health. I suggest holding off on any roughhousing until you get, ah, a handle on this. Pain and humiliation both have their places, for sure, but even most people who like that sort of thing prefer tops who can control themselves. The ones who can't are not dominants; they are bullies.
In case you don't believe how common a kink the ball-kick is, at least as fantasy material and especially for the ball-bearing half of the population I offer the following examples from my archive, where they have been languishing for lack of (my) interest. Um, happy holidays!
Q: Years back, newly divorced and tipsy, I was at a nightclub and an aggressive woman took me home for some fun. She asked me if I was averse to being tied up. I said no and consented. She led me into her basement, asked me to strip, and then secured my hands above my head. I was excited and it showed. She then asked if she could put a spreader bar on my ankles. I said yes. The ball gag in my mouth made me a bit nervous. After many kicks, knees and hard squeezes, I was delirious. I stayed hard through it all. She then gave me a fabulous BJ. I was swollen and black-and-blue for a month. It has not happened since, but I find myself masturbating to this event years later. Is this type of sex-play common?
A: Of course. What did you pay her?
Q: When I was taking karate, I was paired with a woman to practice no-contact front snap kicks to the groin. She was slow and I made a snide remark. She gave me a funny look, said "OK then," and swiftly soccer-kicked me square in the sac. The pain shot through me and I dropped to the floor in the fetal position. She bent down and said "oops, sorry" but gave me a smug look that said she wasn't. When I got home, my testicles were quite swollen. Then I got an erection I'm not sure why, but the entire ball-busting was a turn on after the fact. The next time I saw her, she smirked and made a snide remark, which got me semi-hard and I was tempted to egg her on again but chickened out. She dropped out of class not long after that. I still fantasize about being kicked or kneed by a woman (no sex). Am I crazy?
A: Since you neither egged her on nor went out of your way later to court injury, clearly not. She was kind of a bitch, though.
Andrea is teaching Sex After Parenthood at Day One Center (www.dayonecenter.com ), Recess (email@example.com ), and privately. Contact her at firstname.lastname@example.org  for more info.