Three earners??

Asian families. Example mom then gf and bf. 3+ earners or could be swingers...

It'd be interesting to see how same sex marriage may have impacted this. In the 1980s same sex couples without kids would have been excluded, now they're included. It's ~1% of all marriages
https://www.census.gov/newsroom/press-releases/2019/same-sex-households.html

In my family of my parents age level, 5/8 have children Of those children, 3/8 have children but 3 of those without are what I'd consider "open to have children", as in no clue if they want/will or not.

In total, 8 couples (16 people) produced 8 people in my parents generation in the family. The followup 3/8 produced 4 so far.

I don't think your point has no value, but I think it doesn't target the right thing to look at, especially since you'd kinda have to consider adoption alongside with it.

I am sure a large percentage of the DINK number is couples living together before having children so this is probably misleading.

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If that were the case you would have as many entering as exiting and the line would be flat. Here you have an increasing trendline.