Middle-class Americans are forsaking marriage amid financial insecurity, effectively making the institution more of a luxury good enjoyed by prosperous Americans.

The middle three-fifths of U.S. earners have experienced the sharpest declines in marriage rates over the past four decades compared with people at the bottom of the income ladder and those at the top, according to a Wall Street Journal analysis of census data from 1980 to 2018, the most recent available. These households earned from $25,000 to $125,000 in 2018....