"Unlike their ancestors in the late 1800s, many of today’s working-class young adults have responded to the difficulty of marrying by living with a partner and having children outside of marriage."
What difficulty in marrying? I could understand a difficulty in living together and having kids. But marrying? That's just a piece of paper. It makes little difference for people who already have a family together.
I think that people simply don't value that paper - it means little to them except maybe an expense for a large party.
Any analysis of marriage rates should consider people who cohabitatate to be married for the purpose of understanding family trends.