I'm guessing those families are not living the conspicuous-consumption that has been the "new-normal" for decades now.
I'll note that it's easier to have a single-income household when you're inheriting a house from grandma, or if dad will help with the deposit for buying a property. However, if both partners are from low-/no-wealth families, they will find themselves forced to get their career on track and/or live a more modest lifestyle than their peers.
As for wealth.. I can't speak to that but we look out for each other and there are several families I know who do not have that kind of support.
> live a more modest lifestyle than their peers.
you mean are less greedy than peers.
I was avoiding speculating on motivations and was aiming for objectivity.
I live between Germany and Portugal, where in both countries is similar to what you describe. It is perfectly possible to raise a large family with only the father having a salary.
Today most couples have the mindset of buying a house for hundreds of thousand euros, pay tens of thousands for a car and also afford vacation to Thailand or whatever frivolous location.
Choosing a more modest location to live, buying second-hand cars without debt, having the wife cooking and taking care of the children is already a huge difference. What is not measured if the happiness of the woman in watching her children grow, that is truly fantastic. Most media favor an "empowerment" rather than presenting as option a traditional lifestyle.
Took me decades to understand this and guide my own family towards a saner lifestyle away from frivolous consuming habits.
In your model the causation is that profiteers caused the breakdown of marriage but we shouldn't confuse correlation with causation.
It's also plausible that the decline of marriage caused the preponderance of profiteers.
Indeed given the modern day 'profit above all else mentality' that seems to pervade many businesses thoughts these days, and the length of history of marriage, I'd say the latter is more likely.
As for dual income. I'm sorry I simply don't believe it. In my conservative Catholic circle in a high cost of living area, I'm seeing many families with almost a dozen kids where only dad is working (my wife and I included). Dad is not even a software engineer making high six figures. In many families the father's are totally normal incomes and we have a fair share of people making below average. Two incomes is a choice. There is no difference in average income between the two groups yet one has different values. One difference is the mother's are very productive amongst themselves getting things done economically.