As a professional psychologist and a former sport doctor I want to express my opinion why female combat is so erotic in men's eyes. I will try to be clear and understandable.
Women like fantasying about men's combat and vice versa. Why? The former fantasy type bases on the quite real biological phenomenon - struggle of males for possession over a female or for primacy (in a herd or on a certain territory). The later fantasy type seems to be less related to the reality. Let's try to figure this out.
If humans and human history are concerned men's combat is probably one of the most common and "routine" activities for all the times. This combat would have the form of duels (armed or unarmed) or numerous warfare acts. For instance, fist fighting and wrestling was wide spread among all ethnicities. The main reasons of men's fighting are self-establishment in the society and possession of a woman. Women subconsciously consider any men's contact combat in such perspective - from sword duel to athletic combat (like boxing and wrestling). I mean not a realized judgment of such an action by a woman but a subconscious instinctive perception of the combat as a fight "over me". A woman's thoughts about a men's combative activity and her subconscious reaction to it might be very different. I noticed that many of those women, who state they hate violent actions and athletic combat activities, stealthily express interest toward such things if think nobody realizes that. A men's single combat is a source of erotic fantasy for many women. I know that because due to my responsibilities I had to attend many boxing and wrestling matches and I talked to female spectators there. At the same time, even though it might seem to be strange but a men's fight is a source of erotic fantasies for men too. It's noticed that a men's combat arouses a man if he sees or imagines a female witness of it. I state that A MEN'S COMBAT IS THE BASIS OF MANY MALE FANTASIES ABOUT WOMEN'S COMBAT. I'll try to substantiate that.
Erotic fantasies are very complex matters where capricious images appear in imagination. Let's consider the situation like this: a man sees or imagines a fight between two other men and a woman watching the fight. This scene has an erotic flavor because it's directly related to sex (the image of males competing over a female for mating should appear the sub-cortex). The imagination of a male spectator transforms the situation: either he places himself in the shoes of one of the fighters or he concentrates on the woman - since she plays the passive role here, his imagination unconsciously seeks getting her the title role. Thus the male imagination replaces the two fighting men with two women while the action keeps running according the same scenario as it has been before.
Besides, the CONTRAST COMBINATION of tender female figures and the manly activity, which they are involved in, is another powerful contrast arousal. In the real life (and in movies) you can see a lot of scenes with men's combat in various forms - from combative sports to duels and brawls. These apparitions are stuck in the sub-cortex and appear being transformed into erotic images. A real female combat is a relatively rare phenomenon and in the most cases it's just artificial and staged for spectators' entertainment. Yet widespread of female combative sports extends the basis for fantasies about this matter. But an image of men's fight over a woman still forms the basis of many male erotic fantasies about female combat. Obvious substitution is made in many entertaining and erotic pictures (innumerable number appears on the book covers and on internet pages): male figures are replaced by female ones while the dispositions and dynamics of the fights remain the same. Women skillfully fence with swords, fistfight, wrestle, etc. The spiciest "male combats in female appearance" are those, where women "participate" in the most brutal acts like gladiator combats.
One more transformation occurs in the male imagination though. I polled several male admirers of female combat. I questioned them which particular participant of a female combat more attracts their attention (more precise - which one more excites). After a thought all of them pointed THE ONE WHO WAS ATTACKED AT THE MOMENT. In other words, a male contemplator concentrates attention at the participant who is undergoes a violent attack. And he definitely includes himself to the action playing the role of the attacker. This observation has certain things in common with the opinion expressed in the article by Alexey Bauman on this site where it was claimed that violence initiates erotic excitements
So, a man watches or imagines a combat action. For example, it's a fight between two men - the imaginer fantasies that that's over a woman and imagines he is one of the "duelists". Or two women wrestle - he concentrates on the violence over one of them, which might be done by him. And so on.
I see here two imaginary replacements: SUBSTITUTION of men's images with female ones in the most "masculine" activity (reaction BY CONTAST) and also IMAGINARY PASSIVE INVOLVEMENT HIMSELF INTO THE COMBAT. In other words, the most "masculine activity" is selected among all conceivable ones - combat, which is also related to sex and the imaginary substitution of men with women is made as well as an involvement of the subject of the fantasy into the imaginary fight.