I don't share grandparent's views of the 1960s, but the cost of additional material goods are trivial.
The main reason you need 2 incomes to support a household is because you assume you have 2 incomes when building the household. In other words, (if 1 income was desirable), a couple lives in way too nice of a house. They also end up with two nice cars instead of one average one and probably eat out more than they could afford on one income. That's pretty much it; all other costs are trivial.
(Practical example: I'm paying $12,000 a year for a room in San Francisco. Car probably runs around $3,500. In comparison, the price of all my aggregate post-1960 technology -- cell phones, computers, internet, etc. is well below $3,500 -- the price of the car.)
It's not extra stuff, it's the price of housing and education. You might be perfectly fine living in a single room in SF, but what if you have a family and actually care about what school your kids go to? Maybe you don't want to live in a neighborhood where there is crime right out in front of your house all day long.
The main reason you need 2 incomes to support a household is because you assume you have 2 incomes when building the household. In other words, (if 1 income was desirable), a couple lives in way too nice of a house. They also end up with two nice cars instead of one average one and probably eat out more than they could afford on one income. That's pretty much it; all other costs are trivial.
(Practical example: I'm paying $12,000 a year for a room in San Francisco. Car probably runs around $3,500. In comparison, the price of all my aggregate post-1960 technology -- cell phones, computers, internet, etc. is well below $3,500 -- the price of the car.)