Heteronormativity is the assumption that heterosexuality is a 'natural' representation of sexual and gender expression in society. Essentially, heteronormativity suggests that the male/female binary is the normal way to express sexuality and gender.
More About Heteronormativity
Heteronormativity describes the idea that any sexual or gender representation beyond the male/female binary of heterosexuality is abnormal. There's no room in heteronormativity for fluid gender or other ideas of gender and sexuality.