Sorry for the poor phrasing, but I'm looking for the name and some examples of the fallacious notion that civilization has always gotten more "progressive" or "liberal" or whatever you want to call it; basically, the idea that over time, a stable culture will always tend toward more sexual freedom, tolerance, etc.
Do you mean like the myth of progress? Assumption of linear progression? Linearity of history?
These are more general ideas in which history is regarded as a constant progression to better, more complex, more advanced, etc. So like women were more oppressed in 1930 than they were in 1960. This is contrasted with a cyclic nature of history or progress, in which the same things keep happening over and over again in a never-ending cycle.
There's the Marxist idea of historical materialism. It states that through contradictions in society, different societal classes come into conflict (class struggle), and over time, this struggle will move society through to new economic systems (see classless hunter-gatherer society -> slave empires -> feudalism -> capitalism -> socialism -> communism) via revolutions by the winning classes. As older economic systems are no longer efficient, new ones automatically take their places, abd bring with them new customs. For instance, slavery is no longer an accepted custom, and as Patriarchy and racism become less accepted, the socialist movement grows, eventually abolishing these along with capitalism.