No fault divorce was first legalized here in California in the late 70s, and in most states by the mid-80s. This increased the number of divorces, and I don't know the numbers, but I imagine there was an initial "boom" of pent-up unhappy marriages that took advantage at first before the numbers leveled out.

There's a reason the sitcoms of the era tended to involve single-parent homes (often with friends/family filling in) - this was an attack on the cultural idea of "norm". Putting children in the role of "man of the family" when there wasn't a father was a big thing, at least in the culture I grew up in. And since everyone now has grown up with that, it isn't going away quickly.

No idea of the quality of this db, but a search for "you're the man of the family" in movies turned up over a dozen matches for that exact phrase: http://www.quodb.com/search/you're%20the%20man%20of%20the%20..., with most being between 1980-2010 - and it looks like those were generally delivered to children (a few to female children).

The Mom may have been head of the household, but the eldest son was (and is) treated as if that Y chromosome imbues him with mystical powers and responsibility, even if very young.