The explanation, that I buy, is that we no longer try to promote good citizenship and good living, and so unsurprisingly people answer the call of the short term, to their long term detriment.

This makes sense if we assume marriage is causal for class. i.e. the people who don't heed the call of the short term and do marry have better outcomes and end up higher class. Choosing marriages naturally sorts people into class, by this model.

Liberals would tell a story where things are reversed and class is causal of the pathology- they would say the economic changes that have occurred for the last few decades have increased 'economic uncertainty' for the lower class (for some measure of uncertainty.) which has lead to marital stress and divorce. Its also worth pointing out that in the lower classes divorce is usually less costly for the man (the wife is more likely to be working at a similar paying job, the man has less stuff to lose)

Personally, I found the book Red Families/Blue Families pushed me away from the first explanation and toward the second (full disclosure, this is part of a larger trend of me growing increasing liberal over the last decade and a half or so.)