When social constructs are changed, we may not see the full results for 50 years or more. If the change was a bad one, the world won't fall apart right away - it will happen over time or generations. In the case of marriage, we'll have to wait for a large number of children of such unions to grow up, and for them to raise families themselves. The jury is still out on what much of Europe is doing.
While (in my own opinion) I don't see any real difference between an unmarried couple raising children together and a married couple doing the same, actual practice may be different. The social institution of marriage encourages couple to stick together to raise their family. I don't have the facts on hand, but it seems very likely that the lack of marriage would translate into more single parent households. And that has been shown to be a bad thing overall - especially in modern society, where there are no extended families around to help out.
There are always exceptions... bad kids from good families, great kids from broken families, particularly devoted/competent single parents.... but the value of social norms is in the averages.
I would like to see comparative rates of drug addiction/juvenile crime/etc. in Iceland over the decades between when marriage was common and now. It wouldn't answer the question, because there are so many variables, but it would still be interesting. Iceland is quite small, as well, so one is never far from an extended family or absent parent.
