Single-parent households have always made less money, and the percentage of single-parent households has doubled to about 25% since 1960. That creates a lot of downward pressure on the income "median household" that is the result of sociological rather than economic factors.
Married couples where only the husband works make almost twice as much now, in constant dollars, as they did in 1950. Married couples where both spouses work make almost three times as much as they did in 1950.
I see some comments in this thread along the lines of "back in the day, middle class people could afford a house, two cars, college for their kids, and vacations." I think this view is more based on mistaken notions of what middle class families could actually afford in 1950. Partly, this is the byproduct of the fact that media always dramatically overstates the living conditions of people at any given income level.
Take, for example, "Leave it to Beaver" which is portrayed as archetypal of a 1950's middle class family with stay at home wife. But the Cleavers are an upper class family! Both parents attended college; the husband drives a top of the line car and has a white collar job. In the show, they have a nice colonial home, a small TV, and one nice car. Today, they'd be a dual-income couple with a McMansion, a couple of BMWs, and several flat screen TVs.