I know this is treading off-topic, but interestingly, "lefty wacko uber-progressive" Seattle (I put that in quotes as it's not my thoughts, but some of the commenters found on The Seattle Times would likely describe it as such) has the highest percentage of nuclear families in the 50 largest cities in the U.S.
I think you and your parent comment may be using two different definitions of "nuclear" though. One definition is the traditional mom-dad-2.3 kids arrangement, but a broader and equally valid definition can include polygamy, gay parents with a surrogate, lesbian parents with a sperm donor, couples where the mom and dad have split and started new families but still have kids in common, and a million other combinations. The most important positive factor seems to be stability, which is easier to maintain when your peer group is accepting of whatever weird arrangement you have. That acceptance is a strengthening factor for the traditional nuclear family as well as the non-traditional combinations.
"Nuclear" should mean that newly married couples form a new household rather than integrating into the household of the extended family.
Why the nuclear rather than the extended family would be considered important for social stability is anyone's guess. We live in an incredibly atomized society that values personal freedom and autonomy over interdependence and social connection, but that doesn't mean "stability".
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.
