I am not agnostic to the view that society has changed because millennials prefer not to have any responsibilities and therefore opt not to get married (a view for which I have great sympathy, I should add).
Young people are more inclined than their elders to view cohabitation without marriage in a positive light. (My grandparents on both sides would have considered cohabitation without marriage a terrible sin.)
As Will Durant said, “A great civilization is not conquered from without, until it has destroyed itself from within. The essential causes of Rome’s decline lay in her people, her morals, her class struggle, her failing trade, her bureaucratic despotism, her stifling taxes, her consuming wars.”