If we're talking about generalizations about history that are assumed to be true but ain't really so much, then there's a lot of this in the popular view of family and gender in the past. ex:
women were only property in biblical times
Augustine, Puritans were anti-sex
forced marriages were the rule before feminism, no romantic love before late medieval times
children just considered little adults in premodern times, no sentimentality because of high mortality
the law/society was never concerned with domestic abuse/child abuse/neglect before the late 19th/20th century
confusing matrilineal kinship patterns with matriarchal societies
You can add to that wife-beating being considered acceptable before second-wave feminism and (even more ludicrously) rape being considered acceptable prior to the Violence Against Women Act on the basis of promiscuity or "asking for it."
