Amatonormativity is the assumption that the traditional view of romantic relationships: a monogamous relationship where the parties are married, live together, and have children in a nuclear household, is the highest form of satisfaction one can achieve in life, and that all people strive for this type of relationship. This way of thinking can be harmful to polyamorous people who may not have or desire a single partner, and to a-spec people, who might not want a relationship like that, or may have a strong, meaningful relationship that is not romantic in nature.
This way of thinking is also one that places certain relationships above others, such as romantic relationships being viewed as being 'above' or 'superior' to platonic relationships. If two people are dating they are 'more than friends'. If they aren't dating then they're 'just friends'. This view is also the reason that queerplatonic relationships are sometimes viewed as 'more than friendships but less than romance', when in reality it's impossible to rank different relationships because it's all relative. Someone may love their friends or queerplatonic partner just as much as someone else may love their romantic partner.
History
The term amatonormativity was coined by philosophy professor Elizabeth Brake in her 2011 book 'Minimizing Marriage: Marriage, Morality, and the Law' where she defines that term as “the assumption that a central, exclusive, amorous relationship is normal for humans, in that it is a universally shared goal, and that such a relationship is normative, in the sense that it should be aimed at in preference to other relationship types.”
Etymology
The prefix amato- is a Latin word meaning 'beloved'.