“Cohabitation, once rare, is now the norm: The researchers found that more than half (54 percent) of all first marriages between 1990 and 1994 began with unmarried cohabitation. They estimate that a majority of young men and women of marriageable age today will spend some time in a cohabiting relationship.
“…Cohabiting relationships are less stable than marriages and that instability is increasing” -The National Institute of Child Health and Human Development
In the United States and in the UK, couples who live together are at a greater risk for divorce than non-cohabiting couples.
Cohabiting couples had a separation rate five times that of married couples and a reconciliation rate that was one-third that of married couples.
https://www.thespruce.com/cohabitation-facts-and-statistics-2302236
As marriage rates have fallen, the number of U.S. adults in cohabiting relationships has continued to climb, reaching about 18 million in 2016. This is up 29% since 2007, when 14 million adults were cohabiting, according to U.S. Census Bureau data.