>That's kind of expected. Our society/economy/marketing are designed to get people to spend 100 percent of their income or more. So when families get 2 incomes it causes inflation or enables people to buy more stuff (iPhones) or whatever until they're spending the same percentage. Aside from not actually saving the extra income, the obvious downside is to screw single people by raising prices without raising their income.
I do not think this has much to do with it.
The much larger sources of volatility would be massive increase in labor supply due to other countries coming "online", women entering workforce (this might be overstated), immigrants entering workforce. And all of that was coupled with huge advances in automation, namely computer software, and then the internet. Although advances in farming and manufacturing automation cannot be overstated either.
Both labor supply and labor demand curves shifted the price of labor down for "unskilled" labor.
The other societal impact that is large is women gaining independence, therefore impacting the couples markets. Due to our biology or sociology or whatever, typically, people are going to seek out mates in equal or higher socioeconomic classes with more economic stability. This results in high earners pairing with high earners, competing with other dual high earning couples.
Finally, birth control reduces need to form families while still satisfying biological urges. I know quite a few people sitting out the family thing because they were unable to find an acceptable partner per their requirements. I think this is being seen in general in all developed countries via the plummeting birth rates.
All in all, people have a lot more information available than they used to, and a lot more control of their bodies, and the conclusion for many is the cost:benefit:volatility ratio of starting families is not worth it.