The key isn't the marriage rate, it's the prevalence of intact families. Scandinavians don't marry, but they do live together and raise children together. The prevalence of intact families (meaning children living with both biological parents) is higher in Scandinavia than it is in the US, even though the marriage rate is lower.

I don't know that people necessarily have to be married and stay married to make the nuclear family work, even if that is the typical manifestation. If two unmarried people have children and contribute to a stable life, then they still are a strong nuclear family even if not bound by marriage. Similarly, if a divorcee pairs up with someone else and doesn't get married, the children can get a de-facto stepparent.

I'm going to see if I can scare up some data regarding families in Scandinavia, but I don't think your point necessarily contradicts the parent post.

With few laws that differential between marriage and people living together, marriage mostly becomes a symbolic ritual. It would be interesting to see how legal systems for alimony or custody of children effects marriage statistics.