In 1960 it was
unthinkable. In 1960 'gay marriage' was an oxymoron. The gay movement
hadn't come out of the closet and the idea of a lesbian was still a
joke that was mentioned in a Tom Lehrer song.
I want to suggest that the issue of gay marriage is related to the end of the elite class in America and its slow disappearance in the other North Sea countries where the idea of gay marriage is prominent.
I suspect that the idea of marriage between a recognized man and woman has been important in the past for the protection of an hereditary class. Bastard is an idea that disappeared about the same time that the elite class was disappearing around 1960, and the same time that divorce became a public sport.
The elite
class needed strongly sanctioned marriage to limit the number of
recognized and supported offspring, which in turn help protect and pass
along all the identifiers of class.
Property ownership and financial inheritance matters were of lesser concern because many more people had wealth than just the elite top class.
By simply mentioning the societies that have well established and long enduring social structures and noticing that they still consider gay marriage an oxymoron I will rest my case: Japan, China, Taiwan, India, Turkey, Saudi Arabia and Mexico.
Sustaining heterosexual marriage is an inherited class issue.