Eren from Attack on Titan has no problems with headbutting Mikasa when she shows willingness to disobey her superiors' orders and later doesn't pull any punches while fighting the Female Titan he knows to be Annie.
In the third Rebuild of Evangelion movie, a situation arrises where Shinji (male) Asuka (female) are forced to fight each other, both thinking that the fate of the world depends on their victory. After he's defeated her and her mecha running out of power prevents her from pursuing him further, she exclaims "You're the worst, you'd actually raise your hand against a girl!" Given that he was willing to talk and explain what he was doing and really agonized over having to hurt a former comrade, begging her to stop, and that she's the one who a) has actual combat experience and b) charged him from behind with a polearm without bothering to hear him out (which actually may have resulted in him finding out that he was tricked by the enemy faction, and thus, avoided lots of the ensuing drama and destruction...), one might wonder wether being male gave him any sort of advantage at all, especially since they were fighting with mechas, not their physical bodies.
Done without lampshading in Excel♥Saga. Il Palazzo shoots Excel with a pistol and a bazooka. This is a rare example of man-on-woman violence being (very) funny, perhaps because Excel herself doesn't make a big deal of it.
Edward Elric from Fullmetal Alchemist has no qualms about hitting girls, much to his younger brother's horror.
Not to mention Roy Mustang when he burns Lust repeatedly until she dies.
Saint Seiya: Ikki does not care for his adversary's gender, and he is perfectly willing hitting his enemy is that person is a woman. It doesn't mean he's happy when someone else hits a girl in his vincinity, and specially if they're bystanders: he was pissed when Esmeralda was fatally struck down by Guilty, and later in the anime filler when little Helen, a 12-year-old girl, was tossed alive into a volcano by Agora and Shiva.
YuYu Hakusho has an instance in the anime where Yusuke reveals he holds this attitude, in contrast to Kuwabara, asking what gender has to do with fighting somebody who wants to fight and declaring there to be no purpose in treating an opponent differently because of their sex. The edited dub plays this straight, while the original Japanese/unedited dub implicitly averts by revealing that the demon Miyuki, the cause of this, is actually a Villainous Crossdresser transsexual, though Yusuke's precise dialogue could be taken as him telling Kuwabara the truth about Miyuki's gender only to keep him from starting a pointless brawl due to being so outraged over how.
There's even been a couple of times where he smacked Botan, once when she nearly blabbed everything about him being a Spirit Detective, and again much later after being mocked too much about his "Spirit Beast" Puu.
In this case, for every time he's hit Botan, there's three or four times Botan has hit him. And the receiver has usually done something to deserve it.
One Piece's Monkey D. Luffy has a notable track record of this. As early as chapter two, he's attacking female pirate "Iron Mace" Alvida. In Whiskey Peak, he and Zoro give Miss Valentine and Mr. 5 an almost Offhand Backhand, and Luffy's the one who hits Valentine. In Alabasta, Luffy hits Vivi after Vivi gets angry at him for not wanting to run around pointlessly. In Amazon Lily, Luffy had to fight the Boa sisters, Sandersonia and Marigold and did so without holding back. He was also fully prepared to fight Hancock. He also recently told Big Mom that he would kick her ass when he gets to the New World.
Most members of the Straw Hat crew don't seem to have any problem with female opponents. Usopp in particular has fought against several female opponents, in one case only losing because Chivalrous Pervert Sanji stopped him from attacking. Zoro spent a large part of his childhood (unsuccessfully) trying to beat a girl, and even berated her for thinking that being a female made her any less capable a fighter. The only other member of the crew with reservations about fighting women is Chivalrous Pervert Sanji.
Trunks destroys all of the future androids without mercy, killing Future Android 18 with an energy blast point-blank.
Super Buu couldn't care less about his opponent's gender; he turns Chichi into an egg and brutally crushes her, and also threatens to kill Videl when she provokes him.
Spopovich is not only very willing to fight Videl in a World Tournament, but he brutally beats her senseless. He would have killed her, but he is forced to simply knock her out of the ring in the end.
Naruto has no problem with this considering he challenged Tsunade to a fight after she insults the Hokage, and mocks his dream of becoming the Next Hokage. Granted, he doesn't manage to defeat her, but in their rematch he wins her over, and manages to get a kiss on the forehead from her, because she admits that his dream of becoming Hokage will eventually become reality.
Tobi also counts considering his fight with Konan, when he actually kills her at the end of it.
And then there's Neji, who almost kills Hinata during their fight after, with some help from Naruto, she shakes off his Hannibal Lecture and fights him back.
It's quicker to point out that Shikamaru is the only character in the entire series with reservations about fighting women, and he does it anyway. (Though he sometimes uses his shadow powers to have the girls hit themselves, like he does to Kin Tsuchi.)
Of course, it makes sense that this trope applies to almost every Naruto character. There are plenty of female ninja in the Narutoverse, so anybody that really Wouldn't Hit a Girl would probably get themselves killed (or fired) fairly quickly.
Sagara Sousuke from Full Metal Panic!. Being a Heroic Comedic SociopathCrazy Survivalist, he displays no reservations towards violence against women. Granted, he tends to prefer shooting at or bombing them instead of engaging in fist fights with them, but this appears to purely be his tactical preference more than anything else (seeing how he's rather physically underwhelming, and has the tendency to rely more on firearms instead of muscle). In one instance, he showed no hesitance in tackling Kaname's female teacher down a flight of stairs. And in another instance, he's shown to put a knife to the throat of an innocent old lady (though Kaname stopped him from doing anything).
Train Heartnet during the beginning of the Black Cat anime, while he was still emotionless and working for Chronos. Right in the beginning, he is shown beating down and assassinating a bunch of nuns (though...they apparently weren't actuallynuns, considering that they pulled out a bunch of guns and tried to kill him) during a mission.
Bandou from Elfen Lied, who proves himself an asshole by immediately back-handing a random female secretary for approaching him from behind, calling her a "stupid bitch" afterwards. In fact, the only people that the man has hit in the series are girls. He even lampshades it at one point, telling a girl that he is assaulting that he really doesn't give a crap whether she's a little girl or not.
Kouta also does his fair share of slapping around any girls he thinks are unruly. In example, when he has a fight with a Yuka in Clingy Jealous Girl mode, he slaps her and then tells her that she's not being fair to him and others.
In one episode of Science Ninja Team Gatchaman, Ken punches a young girl in the stomach. He slaps her later in the same episode. In episode 40 he slaps Jun, the only female member of the superhero team.
In the Bamboo Blade manga, Toyama likes to Kick the Dog by beating up on new kendo club members, and Kirino mentions "Boys or girls, it doesn't matter to him. In fact, I think he picks on the girls even more." Of course, this makes the reader cheer that much harder when Tamaki beats his ass.
In one episode of Rune SoldierGenie is getting increasingly frustrated because Louie doesn't take their fight against a horde of enemy soldiers seriously and always steps in front of her when she's about to attack. She accuses him of doing it because she's a woman, and punches him in the face. To make it perfectly clear that he isn't thinking of her like that, he gets back up and promptly punches her back into the face. After that they continue to beat each other to pulp.
Touma of To Aru Majutsu no Index has no problem hitting women at all if their motivations involve the harming and murdering of decent human beings, and decks one female enemy (who was about to start a full scale war between Espers and Magicians that would result in the death of innocent millions) in the face at least twice in the same arc, knocking her out.
The characters of Hellsing are prime examples, especially Alucard.
Alucard does not hesitate to shoot Seras when she says that she is a virgin. However, the most notable example of this is the brutal murder of Rip Van Winkle: Alucard impales her on her own gun, drinks her blood, and finally absorbs her into his body. He also punches her in the face beforehand. In the TV series, Alucard kills a rogue female vampire without batting an eye. Additionally, he shoots Baobhan Sith to dispel her.
Jan Valentine intends on killing Integra when he and his brother storm the headquarters of the Hellsing Organization. He thinks nothing of it, and even claims that he may "skullfuck" Integra's corpse.
Hei of Darker Than Black is a consummate Combat Pragmatist and has no problem fighting female opponents. Veering into the villain version of this (he's a pretty ruthless Anti-Hero), on several occasions, when sufficiently angered, he has also struck defenseless female characters.
Black☆Star from Soul Eater appears to be this, since he had no problem with punching Maka in the face when she told him to Hit Me, Dammit!.
Despite widespread Fanon to the contrary, Ranma Saotome of Ranma ½ is quite willing to fight girls, though he does say once or twice that he doesn't like to do so and, for various reasons, goes easier on them then guys. For example, the whole mess with Shampoo started because he challenged her to a fight and then kicked her in the head off of the challenge log (and if he had been thinking to change back to his real form first, he wouldn't have gotten the Kiss of Death). He usually doesn't get to fight women particularly seriously because the matches he gets into opposing them tend to be goofy Martial Arts and Crafts stuff, and even if this contest involves physical combat he'd rather defeat them by ring-out or Defeat by Modesty than by actually hurting them. When push comes to shove, however, and the woman in question becomes a real threat to his or his friends' safety, he's perfectly willing to fight hand-to-hand with everything he's got —or bury them in a rock slide, whichever works.
Recurring opponent Pantyhose Taro outright says that he has no problem at all hitting girls. However, this means that he fights them exactly as he fights men —spending only as much energy and effort as necessary to defeat and subdue them. Therefore, after kidnapping Akane, he easily disarms and overpowers her with a few well-placed strikes; when fighting Rouge, he goes all-out, whether she's in her demure human form or in her psychotic Ashura form.
Fairy Tail characters in general seem not to see sex or gender as an obstacle to beating the crap out of people. Occurs most notably when Psycho for Hire-turned-Sociopathic Hero Gajeel spends several gleeful minutes slapping a bound and helpless Lucy around just for the hell of it, but even then no one seems to care that Lucy is a girl so much as that she's chained to a wall.
Neuro is just fine hitting Yako, breaking her neck, throwing her out the window, using her as a chair, and just generally making her life hell.
Most of the characters in Mahou Sensei Negima! don't have a problem hitting women. Yes, most of the heroes arewomen, but their enemies are happy to fight back, and the main (male) character doesn't flinch at beating up female villains. The one exception is Koutaro, who is a bit of a Technical Pacifist when it comes to women (he'll knock a female opponent out of the ring with the air pressure from his punch, but he won't actually hit her), but everyone else treats him like an idiot for making such a stupid lifestyle choice.
In chapter 317, Fate Averruncus is shown slapping a bound Nodoka (because she made him regret he didn't gag her as well)
For Guts in Berserk, there is no such thing as chivalry when he deals with female opponents. But then again, holding back against women like Rosine or Slan would be plain suicidal.
His first on-screen kill is a female demon after getting it on with her, sticking his Arm Cannon straight down her throat before blowing her brains out when she transforms to eat him alive like she previously did to Corkus during the Eclipse.
It is played straight during the early flashback arc. Future love interest Casca attacks him after finding out he had to strip her naked to treat her wounds, and he tells her that he'd knock her on her ass if she wasn't a woman. Of course, Casca's far less of a threat to him than demons, and it was more of a fit than a fight.
Umineko no Naku Koro ni: Both Battler and Hideyoshi, according to the third arc. To be fair, both girls (Beatrice and Eva-Beatrice, respectively) were being rather mean at the time.
The Furnitures have no real qualms about the genders of their rivals during their fights, either. i.e., the Goat Men from the second arc kill Rosa and Maria after the first goes Mama Bear moment.
Kyon tried to punch Haruhi for going way too far in messing with Mikuru.
Agon of Eyeshield 21 has this as part of his establishing character moment. When Mamori turns down his advances he shoves her to the ground.
In Gundam SEEDKira Yamato slugs Cagalli after she gets a bunch of her friends killed, and then tries to belt him when he calls her out on it. Muruta Azrael has no problems with slapping a girl (Flay) for not obeying his orders. Or shooting her a half-dozen times in the gut (Natarle). Neither does Rau Le Creuset, who kills Flay as well as others.
In Gundam SEED Destiny Kira ends of the being the one who kills Stella, though whether he was aware of her gender is up in the air. During the finale, neither Neo nor Athrun has a particular problem with tossing Lunamaria Hawke around like a ragdoll, and Anti-Villain Shinn Asuka makes a determined effort to kill Cagalli Yula Atha during their one fight.
The world of Pokémon Special has no gender issues as evil guys have no problem attacking heroic women and good guys have no problem attacking evil women. It helps that it's (usually) the Pokemon themselves that are doing the attacking, but it's worth noting that the girls take just as much damage as the guys.
Brock actually hits Misty twice and comically in the episode "Bulbasaur and the Hidden Village" when she's teasing him about having a crush on Melanie. She gets him back in the next episode, kicking him in the face after a fake bug scare.
Baccano!'s Claire Stanfield considers himself an equal rights activist: he fights for the right of people of all genders to have their asses kicked by Claire Stanfield.
In Gundam Wing, Dorothy Catalonia tries to InvokeWouldn't Hit a Girl against Heero Yuy during a fencing match. He quickly proves her wrong with a thrust that snaps his foil like a twig and embeds it in Dorothy's mask about an inch away from her face.
So it seems Heero was all talk after all.
In Bleach, Ichigo introduces himself to Rukia with a foot to the back. Generally, if males go to fight females, few guys will show much hesitation into fighting them; a good example would be Yamamoto against Halibel's Amazon Brigade, whom he almost torches alive. All of the villains apply, especially Grimmjow and Yammy.
Grimmjow stabs Rukia with his hand to see how strong she is. He also ambushes Loly and Menoly when they are torturing Orihime: he kicks Loly in the stomach and tears her leg off; when Menoly attempts to attack him with a cero, he counters with one of his own and incinerates her upper body, killing her. Later, when Orihime questions him in the middle of his and Ichigo's fight, Grimmjow grabs her in a Neck Lift and almost chokes her; he also threatens Nel with kicking her if she interferes.
A good part of this crosses into Would Hurt a Child. For all effects: Loly and Menoly have the mentalities and looks of kids under 14, and Nel is a little girl Arrancar no older than 8 unless she's into Big Nel mode, that is..
Grimmjow's got nothing on Nnoitra. He Would Hit a Girl especially if she was stronger than him, due to the belief no woman should stand higher than a man on the battlefield, even if he had to cheat to win. Nel learned this the hard way. Probably doesn't help that everything about the guy is just, well, creepy.
Yammy is just as ruthless. In the episode he first appears in, he smacks Orihime so hard he sends her flying, and then aims a cero at her and Yoruichi. In a later episode when an arrancar nurse finishes restoring his lost arm, he slams her into the wall with such force that she is killed instantly (in the manga, he crushes her head with his fist). Even later on he attacks Loly and Menoly for no reason whatsoever, first punching Menoly so hard she crashes through the wall and then beating Loly to near death.
Discussed by Jackie and Renji when she asks him if her gender is a problem for him in their fight. Then Renji zigzags the trope: he tells Jackie as he gives her a Curb-Stomp Battle that gender does NOT matter in the battlefield, but then refuses to finish her off because of her being a female.
Okabe Rintaro of Steins;Gate punches an emotionally disturbed Moeka right in the face in episode 19.
Discussed in the manga The King of Fighters: KYO (based on the KOF games). When Athena tells Kyo's girlfriend Yuki that she lost in a fight to Kyo and that's why she transferred to their school, Yuki is pissed at Kyo for hitting a girl (and one younger than both of them), which actually offends Athena since she thinks Yuki sees her as Just a Kid. Played more seriously later when Yuki tells Iori "Go Through Me" when he insists on fighting Kyo and he takes her hostage as a reply — but when Kyo says "You Wouldn't Hit a Girlwho can't fight back!", he agrees and lets her go.
More than a few cases involves females being murder victims or getting hurt, whether they are Asshole Victims or not. Several cases stands out, one features a young girl named Minako got killed because she knew too much, as she found out that her 2 teachers were involved in a fraud and they strangled her to death to make it look like she committed suicide. Another case involves a man impersonating his dead friend and business partner stabbed his sister-in-law Machiko to death. It was self-defense, though, since Machiko tried to kill him so she can get her husband's insurance.
Black Butler: Dark Action Girl Grell Sutcliffe kills Madam Red when she refuses to harm Ciel, stating that she is "disappointed in her" for being so soft. She also writes Matilda's name down on her 'death list' for sleeping with Sebastian.
Alois also has few qualms with physically punishing his maid Hannah, even taking out her eye.
Being a World of Action Girls, the few male battle aliens in Sekirei have absolutely no issue with fighting women seriously. On the other hand, two different Jerkasses are also shown to consider hitting women for annoying them to be perfectly acceptable. Both get what's coming to them courtesy of Yukari and Shiina.
Nako's manipulative uncle in Poor Poor Lips, who strikes Ren in the face when she stands up for Nako, with the entire sequence framed like a scene of domestic abuse.
It should be noted though that while most of the people he hits are girls, when it's time for a guy (namely Isshiki) to get smacked around, Yuuta pulls out all the stops and just wails on him. Getting chopped or slapped on the head is nothing compared to being comboed.
In Bokurano, Jun Ushiro has a habit of hitting his sister Kana, which greatly disturbs the other Zearth pilots, especially Maki (who believes that he's using their being related as license to treat Kana that way, not knowing that they're not related). Kana's gender is less of an issue than her being younger than Jun, being his sister and not doing anything to deserve it.
Almost played straight by Ryoma in the anime, too. At some point during the Jyousei arc, Ryoma is practisting his shots on his own; he then heas noises and sees that a local Alpha Bitch and her Girl Posse have not only pushed Sakuno to the ground, but are now threatening to do the same to Tomoka. He then stops his practice and throws a tennis ball right at the Alpha Bitch so the group will stop harrassing the girls — had said Alpha Bitch not been fast enough to dodge, she would have been hit point blank.