Certain ideals of sexuality and gender roles are expected to be embodied by males and females in fiction in order for the characters to be attractive to audience members of the opposite sex. This is a consequence of Men Act, Women Are applied to sex appeal. That is, a man's attractiveness is a result of his behavior, while a woman's is due to her passive physical attributes. Sexy women are physically desirable; sexy men are physically strong and protective.
For male characters, strength and acting dynamically is "beauty." Passivity is ugly. Facially ugly yet dynamically active male characters are more attractive than their physically weak yet facially pretty counterparts. To be sure, dynamic, evil men are more likely to be attractive than their static and kind counterparts.
For female characters, passivity is "beauty." In extreme cases, female characters who are very active will be undesirable or will be shown to only love a man stronger than themselves. Alternatively, a physically unattractive woman will always be unattractive regardless of how proactive she is or the moral value of her actions —her personality and her morality will always be outweighed by her looks.
Essentially, physical attractiveness only ever adds to a woman's sex appeal, while being active may detract from said appeal. Whereas a proactive personality only ever adds to a man's sex appeal, while being facially pretty may detract from his desirability.
These different standards lead to the genders being held to equally-damaging (though different) standards of attractiveness. As expected with a name like this, this trope leads to numerous Unfortunate Implications.