"For instance 2-parent households will either be able to provide a much higher average income in the case when both parents work, or a more supportive environment when only 1 parent works."

I knowed a single-mom-family of 20 (twenty persons most under 21y), so if that mixed comment seem to look like 'that 2 parents build a safety-net for their children, if one becomes ill - for example - no the heck if you think how worse it is when a child got ill, 'horrifying!', think in that family of 20, on a regular basis _all_ got ill. At the same time. So you've to get a complete medical "lazarett"-team (doctor, sisters, helper,...) but to underline it (for the extreme... with the 'working parents') or there may be a need for an nanny-state-scenario... that may be called 'social'...

And for the original question: "Why would marriage matter", cos marriage seem to be often about rule and expections, not? Um But if i remember correctly, there were some academics, centurys ago speaking about "Moralstats" ("Moralstatistik" in German), where one finding was that not been married correlates with bad-tooth.

regards,

And while correlation is not causation, many of these factors are obviously causal. For instance 2-parent households will either be able to provide a much higher average income in the case when both parents work, or a more supportive environment when only 1 parent works.