the theme can be interpreted as being sexuality because Esperanza tells the stories of the other women in her neighborhood who are victims of domestic abuse, etc. which deals with gender issues. In addition, she is becoming a teenager in the story for example the part where she wears high heels and walks down the street and boys stare at her and her friends, etc.
basically, the theme is not really sexuality, it is gender. Esperanza didn't want to grow up to be like the other submissive women on mango street (she wanted to own her own house, not have to depend on a husband, etc.) and thats pretty much what the story concludes.
