hm.... I think one of the big problems is in the nature of combat. Fighting and sex tend to come from similar emotional places, and they're the two most physical interactions you can have with another person. I think a lot of writers don't know what to do with this, and they feel the need to sexualize the woman as a fighter. I don't think this is (typically) intentionally done, but I think it's one of the big problems that leads to how women are portrayed, and the kinds of costumes they tend to wear. I think a lot of people have a hard time visualizing women who are sweating, panting, and flailing their bodies into all sorts of contortions without thinking of sex, and sexualizing it from there.