A guy that has no problems with violence against women is treated as morally questionable at best or evil at worst in a work's narrative and other characters.
If he's good, it's either because he's just not sexist and he believes in beating people up equally, or because he knows that holding back against female opponents is a good way to get his ass kicked or killed or endanger his friends/allies/muggles. Alternatively, it could be the victim who tries to argue that he shouldn't hit her because she's a woman; the narrative depicts the claim as wrong, allowing him to proceed. In all these scenarios, he will stop as soon as she's no longer a threat.
If he's evil, it could be done in a multitude of ways. On the best end of the scale it could just be Equal-Opportunity Evil, like fighting Batgirl because she's a threat, and makes her look stronger when she beats him. More unpleasantly, it shows a Dirty Coward, attacking those weaker than him, and/or he's a bully who likes to make people afraid of him, or at the worst end of the scale, he's a sick monster who gets his kicks from hurting women. Ultimately, for it to be played as a villainous deed, it is necessary that the woman in question is not a threat to start with.
Naturally, modern female characters tend to go into battle fully expecting to be hit. If anything, many of them would probably be insulted if a male opponent went easy on them because of their sex. That said, an unprovoked attack on a girl, especially one who can't fight back, is still often played as a Kick the Dog moment, more so than if the genders were reversed. In milder works it is perfect as a Moral Event Horizon without the need for gore, torture, rape or death — it shows the villain will even hurt those who pose no threat.
This can often overlap with What Measure Is a Non-Human?, where even a hero who would not usually hit a girl has no qualms about it if she is some kind of witch, alien or whatever. In these cases, it may be more or less justified, depending on how obviously inhuman the female enemy is.
The opposite trope is, of course, Wouldn't Hit a Girl. When hitting girls is Played for Laughs it can be a case of slapstick. Obviously not present when a woman hits a woman. If the circumstance is designed so that women are only hit by other women than it is a Designated Girl Fight.
