Nowhere in the article is mentioned the obvious common denominator - women are no longer utterly dependent on their husbands. People didn't stay together more often in the 50s because the couples were better, or because people were held to higher standards, it was because getting out of a terrible marriage was incredibly difficult if not impossible, and even if successful would cause immense hardship.
The article also simply takes for granted that it's a bad thing that these nuclear families do not stay together. While it's true that just directly comparing the children of single mothers to those raised by two parents the former tend to be worse off by a number of metrics, this disadvantage disappears once you normalize for the socioeconomic conditions of the mother, and indeed they are actually slightly better off.
People aren't irrationally making their lives harder, they take actions they feel will be beneficial, and it's unsurprising that they have a good idea of what would be personally beneficial. While the exact mechanism is up for debate, the leading theory is that in underprivileged communities where people tend to have children at a young age a heavy reliance on multi-generation family is more beneficial than the nuclear structure (or in other words, when you have a kid at 18, your 40 year old mother can probably provide a lot more resources and experience than a 19 year old guy).
You can't simply say that men aren't stepping up; the world has changed, mostly for the better, and as a consequence family structures are going to look different now from what they once did.