> That's missing forest for the trees. The more important part is that average 3.7 people household was a single-income one.

That's not necessarily true. Per FRED (https://fred.stlouisfed.org/series/LRAC25FEUSM156S), in 1963 about 44% of women aged 25-55 were in the workforce. That's obviously much lower than the ≈96% rate for men, but it implies that the average couple had closer to 1.4 incomes. The comparable rate based on today's labour force participation rates would be about 1.66 incomes.

The single-income family may have been the norm, but it was far from a rule. I think that our popular perception of life in the 60s has been filtered through media portrayals, which all tend to reinforce ideals as commonplace norms.