Related terms include cisnormativity, heterosexism, gender binary, gender essentialism .
"The paperwork for my kids' school has a space for contact information for 'mom' and one for 'dad.' That's so heteronormative."
Heternormativity is the belief or assumption that all people are heterosexual, or that heterosexuality is the default or "normal" state of human being. It tends to complement and accompany concepts like cisnormativity, gender binarism, and gender essentialism. A heteronormative society operates on the assumption that heterosexuality and specific gender features are the human "default." These assumptions can be hurtful because they are stigmatizing and marginalizing, making people who are LGBT+ feel like they are perceived as deviant or unnatural.
The concept of heteronormativity can exist on both a societal and individual level. On a societal level, heteronormativity takes the form of denying marriage equality and same-sex adoption. On an individual level, it can take a form of unintentionally inaccurate assumptions-- such as assuming that a woman is referring to a man when she mentions a spouse or fiance.
The term heteronormativity traces its origins to the early 90s, when it was popularized by Queer Theory expert Michael Warner in "Fear of a Queer Planet." It is a portmanteau of "hetero-" meaning opposite, as in heterosexual, and "normativity," meaning a system of normative assumptions.