well, you can't concretely pinpoint when a culture decided that something like gay or interracial marriages were okay. it's a huge, long grey area pinpointed by thousands of events, like "sidney poitier portrayed engaged to a white woman in successful movie" and stuff.
but the way social change tends to happen is this:
a sufficient number of educated, active, or important people come to understand that a certain thing is wrong or right that wasn't considered wrong or right (or, not to such a degree) before.
those people lobby lawmakers for change. they also prevail upon the generally static and non-progressive populace to accept the new way.
soon, pressure from the activists combines with falling political pressure from the lawmakers' constituents if they (the lawmakers) accept the new way. new laws are passed. (crucially: without the pressure from the activists, MUCH more societal acceptance would be necessary. one enlightened activist counts for one hundred or one thousand enlightened people)
the laws are on the books, and the large majority of the populace goes about slowly converting to accept the "new" way.
