It's one of those terms that are loosely defined (at least in certain societies) and mean very different things to different people.
Is an electrician earning US$80k/year middle class because he's well compensated for his work, or working class because he's doing manual labour?
In the 90s, I remember "family values" having been used by politicians as a polite euphemism for the very narrow "straight, Christian, atomic" view of what a family should be.
I'd definitely view a lack of divorce as a pretty big part of it.
But as you said, it's entirely personal and subjective. :)
% divorced among those married, by age 46:
Did not graduate from high school: 58.8
High school graduate, no college: 49.1
Some college or associate degree: 48.5
Bachelor's degree or higher: 29.8
https://www.bls.gov/opub/mlr/2013/article/pdf/marriage-and-d...