Assumption that every person should be heterosexual, thus marginalising those who do not identify themselves as heterosexual.
Additional notes and information
It also implies that heterosexuality is superior to homosexuality and other sexual identities and orientations. Thus, heterosexism is about power rather than sexual orientation.
Mediterranean Institute of Gender Studies (2009). Glossary of Gender-Related Terms. Available at: http://www.medinstgenderstudies.org/wp-content/uploads/Gender-Glossary-…