The story "Boys and Girls" by Alice Munro is about societal differences for different genders, and the horses' genders are no exception. As a child, the narrator imagines stories about herself that feature "courage, boldness, and self-sacrifice," but her real world does not afford that opportunity for women. The narrator helps her father around the farm, and feels proud of it, but she is still put down by other characters, such as the salesman who refers to her as "only a girl". Her mother stays in the house most of the time, presumably doing "women's work," but the narrator looks upon the house as a prison -- the prison of women's oppression. She is shocked when she overhears her mother saying that her brother will be a real help one day -- she has been helping him all this time, and her brother is still basically young and useless. Her mother also voices her plan to use the narrator in the house more, because right now, "it's not like I had a girl in the family at all". As the narrator learns more about what is expected from girls in the world, she likes it less and less.
Mack and Flora are old horses that are going to be shot, and the narrator forces her brother to witness Mack's shooting. She is angry deep down with her brother for being lucky enough to be a boy in her society, and the fact that Mack is a boy is definitely important and relevant. When it's time to shoot Flora, the narrator opens the gate instead of shutting it, setting her free. Flora's gender is important to the story: she represents all of femininity to the narrator, and setting Flora free is the narrator's last, desperate attempt to set herself free. In the end, she is "absolved and dismissed" with the words, "she's only a girl," having failed to set either Flora or herself free from society's oppression of women.