What is heteronormativity? Heteronormativity is, according to Wikipedia, "the belief that people fall into distinct and complementary genders (man and woman) with natural roles in life. It assumes that heterosexuality is the... norm." In layman's terms, it's the assumption that everyone is straight and cisgendered (the gender they were assigned at birth), and that you can only be one or the other, straight or gay. One might assume that the queer (LGBTQ+) community wouldn't have issues such as this, but we truly do, although it's somewhat different than the way it is in the rest of the world.
Within the queer community, it is erasure. I identify as bisexual, and I've had people tell me to my face that those who identify as bisexual are just doing it for attention. People assume that bisexual people are just "curious" or "experimenting," and this isn't only people who identify as straight. I've had gay people tell me that identifying as bisexual is just a phase, that you can only be straight or gay. The term for this is "bisexual erasure," but also applies to identities such as pansexual. It's incredibly frustrating to be told that your identity isn't true, that you're a "curious heterosexual."
I've also noticed that my asexual or aromantic friends have similar issues with erasure. It's assumed that people who identify as such (asexual meaning not interested in sex, and aromantic meaning not interested in a relationship) aren't valid. They are told that they "haven't met the right person," and it's heavily implied that they'll end up in a heterosexual relationship. This sort of rhetoric implies that relationships with heterosexual sex and marriage are the only good thing in the world. It's harmful, and it isn't something heard only from non-LGBTQ people. It's heard from queer communities, telling ace-spectrum people that their feelings aren't valid. Invalidating feelings are never okay.
It's truly frustrating to deal with heteronormativity from anyone, yet it is a different kind of hurt when dealing with it from people who are a part of the community that you identify with. It hurts in a different way. It's one thing to not be accepted by people you knew might not accept you, and a complete other thing to not be accepted by people in a community you consider yourself to be a part of. So please, if you're reading this, no matter who you are-- Accept love in your life. Accept the people in your life. Accept that you can't stop them from loving who they love, and that it isn't your place to. Let love in to your life, and stop trying to stop people from loving.