I am 40, and it was a nostalgic conservative thing among my parents' generation. For my parents and their fundamentalist subculture, it was almost a religious obligation, tied up in ideas about gender roles.
I'm more than a decade younger than you and grew up in a single income family, lots of children, solidly middle class, and neither of my parents had beyond a high school education. My dad was a straight C student through school and neither of my parents could help me with my math homework by the time I was in junior high...so hard working people, but not book smart by any means.
Lots and lots of my school mates families were similar.
Northwestern Pennsylvania. Rust belt, Great Lakes region.
Anecdotally, more than half of my peers had both parents working. My mother and both grandmothers had jobs. The attitude was that two incomes are better than one in an age of economic uncertainty - an era that kicked in no later than the 70s.
I'm pushing 40 (upbringing: United States, middle class, medium-sized city) and have never, ever heard of this dream of a single-income household.