What is 'heteronormativity? According to the Meriam-Webster dictionary its "of, relating to, or based on the attitude that heterosexuality is the only normal and natural expression of sexuality" But we live in a complex world where this assumption that someone must be heterosexual can be wrong, demeaning or harmful. The reinforcement of heteronormativity and heteronormativity virtually erases the entire LGBT+ community. Here are just a few reasons why heteronormativity is so harmful, and if you reinforce these ideas SPOILER ALERT: You're a bad ally.
1. It reinforces Gender Binary.
With heteronormative ideas comes the belief (whether you believe it is implied, subtle or blatantly stated), that only two genders exist. It paints the issue as very black and white-that someone is either male or female. But this is not the case. Reinforcing the gender binary is incredibly transphobic to say the least. It completely denies the fact that gender (like sexuality!) is a fluid and dynamic spectrum. If you want to learn more about the massive spectrum of gender check out a great resource here. It has various non-binary terms and identities with their definitions.
2. In sex Ed its DANGEROUS
LGBT+ populations are virtually excluded from all forms of sex education in schools. This doesn't even account for sex ed programs where information is outdated, wrong or programs that are abstinence only
3. It tells entire groups they don't exist.
In a heteronormative society this is all people think exists: male attracted to female. female attracted to male. There is so much more to the world that heterosexuality. For a really cool and informative graphic on explaining the spectrum of sexuality, check out this great info-graphic here.
4. If you aren't heterosexual, it makes people assume you "became" something else.
Which obviously you didn't. You were always what you were. Maybe you became more open to sharing with people or decided to be who you want to be not what others do, but it was never a conscious decision to be something else.
5.Heternormativity assumes that only one sexuality actually exists (heterosexuality).
Anything different is lumped together, and this "other" category leaves out so much because it is "everything else." This otherness explicitly excludes people from feeling loved and accepted by society, when in actuality the only thing wrong with this scenario is the assumption that there is something "wrong" with being different.
6. It assume anyone who identifies as something besides heterosexual had to "come out"--and that everyone reacts badly.
I know so many people, who the first question they get after telling someone their sexuality is "how did your parents take it?" While its obviously great when parents accept their children no matter how they identify, the concern should not be placed on them. It assumes that people expect parents to disown or abandon their children because they aren't heterosexual. 
7. LGBT+ representation in pop culture is virtually non-existent.
And if it does, TV shows and movies often play to stereotypes. Yet they will say "not all heterosexual people are the same." Then why assume all LGBT+ people are the same?
8. It justifies and reinforces dangerous ideologies and practices.
Up until 1961, homosexuality was a criminal offense on a national level in the United States. Following this, Illinois became the first state to decriminalize homosexuality. It took until 2003 for homosexuality to be decriminalized at the federal level. Up until 1973, homosexuality was even considered a "mental disorder" by the American Psychiatric Association. Unfortunately, practices like "Straight conversion therapy" still exist, although many states are working to pass laws outlawing it. Hopefully it takes less than 42 years for it to become illegal on a national level.
We do not live in a black and white society, why should we be governed like we are?
