Heterosexism is when people think that being straight is the only way to love or be with someone. It's like when a big brother or sister says, "no, you can't play with us because you're too little." Some people think that only boys should like girls and only girls should like boys. They might make fun of or be mean to people who like someone that isn't the same gender as them. It's not nice to be heterosexist because everyone should be able to love who they want without being judged or put down because of it.