I don't know if that is true. I would say we are solidly middle class (our neighbors on our street include a paramedic, an elementary school teacher, a hospital receptionist, a paralegal, a yoga instructor, a non-profit office manager, etc.). My wife is a stay-at-home mother to our 7 children (though the 2 oldest are away at college) and hasn't had a paycheck since we were in college. We have 2 cars, a house with all the automated amenities, etc. I just work as a programmer making about $100k a year. After taxes, we live on about $4k a month.

This isn't fully true though. 'Middle class' means something vastly more dramatic than it did in 1959. Homes are 2x bigger, 2x nicer, they are fully automated (washing machine, dishwasher, microwave), we have internet, cellphones, 2.5 cars per home instead of 0.8, we travel farther and more often, there's infinitely more variety in food and every other commercial product etc..

Aside from real-estate - a family could live on one income if they wanted to give up all of those things.

I'm not entirely disparaging your point, other than to say the entire system has changed, it's not like 1950's standards now cost 2x.

But yes, surely it's a weird artifact of classical liberalism that 'freedom' is somehow implied to be 'now you have to work for money or starve'.

Edit: I should add, that by any economic definition, if during one era only 'one parent' is working, and during a different era 'both parents' tend to be working, it's just a mathematical reality that those with only 'one parent working' will move down the economic ladder relative to others. This would be true before a single sociological artefact contextualised the fact.