[ajju]

Where we probably agree:

I would agree that it is the commitment that matters.

Where we may agree:

Social norms really do impact human behavior. Marriage is a social norm supporting long term commitment. In communities where it has been replaced with another social norm supporting commitment (eg my well-off friends in Europe), it has become less relevant.

I also posit that adults in committed coparenting relationships constitute a small minority of unmarried adults in America (vs. France for example where a majority of my friends with kids match your description).

Where we probably disagree:

In my observations of close friends in loving relationships with children, previously in loving marriages, are now divorced and in respectful and functional coparenting but not cohabitating relationships.

For a considerable amount of time, they are functionally single parents. In most cases parents and siblings of one ex-spouse are unlikely to want to support the other ex-spouse with in-person child support.

The bright exception to this rule seems to be divorced co-parents who live in close proximity or in one instance in the same duplex and are good friends.