The "low blow" is one of the oldest Heel tricks in the book. Groin attacks are supposed to result in a disqualification, but it doesn't stop heels from sneaking them in without the referee noticing. Or, occasionally, getting deliberately disqualified by using one (championships usually only change hands if the champ is pinned or gives up). Ric Flair was particularly adept at these.
Surprisingly, Ric Flair doesn't seem to cut down on the groin shots when he's a face.
Chyna used the low blow continually during her Heel run. It never seemed to get her disqualified.
In general, professional wrestling tolerates groin attacks by women on men. Its rarely accepted the other way around.
Female wrestlers have the same reaction to receiving groin attacks as male wrestlers; after a match where she was straddled on the top rope, Jacqueline once said "you have no idea how much that hurts a woman."
Stone Cold Steve Austin in his wrestling attire would sometimes hold his downed opponent between the legs and stomp him in the family jewels just because he likes to open a can of whoop ass.
The Dudley Boys had the "Whazzup Headbutt", a flying headbutt to an opponent's groin delivered by D-Von. They performed this move on Stacy Keibler as well.
Goldust had the "Shattered Dreams", where he put an opponent in the turnbuckle, put their legs behind the ropes to give him better access, and then kicked them.
The Toryumonstable Crazy Max had a move that went like this. Two of the C-Max members would each grab an opponent's ankles and hold him upside down and spread eagle. TARU (who was a trained martial artist outside the ring and used lots of martial arts kicks in his matches) would then deliver an axe kick to the groin.
Ironically, though, a few 'legal' variants exist. These are considered legal because the groin is theoretically not the area being targeted by the moves.
The Manhattan or Inverted Atomic Drop, where a person is dropped tailbone first onto their opponent's knee.
A belly to belly position which causes the opponent's leg to hit the victim's groin.
Hardcore Holly's hardcore kick, where he hangs an opponent over the top rope and kicks them in the abdomen, causing his shin to hit his opponent's groin area. This is justified due to being an indirect strike to the groin, as the target of the attack is the abdomen instead.
A rather common variation happens when a wrestler positions himself on the top turnbuckle. If someone moves his legs or jars the rope hard enough for him to lose balance, there's probably a 9/10 chance that he's going to land on his nuts. Announcers used to call attention to it, but even when they don't, the way the wrestler tends to sell the fall makes it pretty obvious.
Many ECW valets go for a low blow against men when interfering in matches, but Luna Vachon once took this to the extreme by grabbing Stevie Richards by the testicles and tossing him across the ring.
Raven would often start off No Disqualification matches with a blatant low-blow, playing right into his Evil Genius gimmick.
Torrie Wilson once did this to Perry Saturn when they were the final two competitors left in an elimination match. Didn't work well enough to get the pin and then he gave her one right back and won the match.
Perhaps the most infamous example in wrestling history occurred in late 2005, when the entire McMahon family was in a rotten mood (all four members had been stunned by Stone Cold Steve Austin the week before) and they were determined to take out their anger on everyoneexceptAustin. Stephanie slapped ring announcer Lilian Garcia just for looking at her, Vince temporarily hired every single person in North America to work for the WWE just so he could fire them all, and it was vowed that someone was going to get terminated that very night, leaving Eric Bischoff a nervous wreck as he was sure he was going to be the scapegoat. The climax of the evening was Vince, Shane, and Stephanie bringing Jerry Lawler and Jim Ross into the ring to force them to apologize for what had happened, even though neither of them had had anything to do with it. Lawler gladly apologized, but all J.R. would agree to say was "I'm sorry your wife got stunned." Shane and Stephanie immediately began throwing tantrums, but then Linda McMahon - usually the Only Sane Man of the family - came out for the first time that evening and told her children that instead of throwing a fit when something didn't go their way, they should actually do something about it - and demonstrated this by firing J.R. on the spot (it was just a Ten-Minute Retirement, anyway) and then kicking him in the crotch. (In 2010, when Linda was running for the U.S. Senate from Connecticut, The Daily Show played a clip of this very scene, and one of Jon Stewart's correspondents jokingly demanded to know why no one had cited this incident in an attempt to smear the McMahon campaign until then.)