Some argue there is a "Two-Income Trap" where having two incomes did make certain things more affordable for the middle class like housing but eventually it simply drove up the price of housing & other goods. So now a dual income is simply normal and only having a single income is a disadvantage. Therefore it's not super surprising to see single young adults moving back home.
In certain super high CoL areas like the Bay Area we do see a bit of a trend towards 3+ communal living (post college) in the form of group homes. While this decreases the burden of real estate prices, in most cases residents are not building up equity in the home since they rent. In some cases they may be saving enough to offset the lack of equity but that's probably not very common.
It’s largely an outgrowth of extreme prosperity and urbanization.
Conversely, you don't need to be married to be in a relationship with two incomes.
That's true. But a full-time homemaker usually generates economic value that saves money in a long litany of ways. Staying home with kids saves on childcare. Being able to cook meals saves on eating out. Laundry and cleaning saves money on housekeeping costs. Not to mention all of this labor is accumulated tax-free (i.e. if you work to pay for some expense, you have to pay tax on the income first.)
> Conversely, you don't need to be married to be in a relationship with two incomes.
You absolutely don't. But the statistics are unambigious. Unmarried couples are substantially more likely to split than married ones. The dissolution of a household tends to be an expensive affair that permanently impedes wealth accumulation.
Not to mention that because of this, unmarried couples are substantially less likely to join their finances. Therefore they tend to get significantly worse deals than couples with joined finances. For example two-incomes who jointly sign a mortgage can get a much better APR, than when the mortgage is in one person's name.