Cohabitation is a term usually used to describe a couple living together in a sexual relationship without the benefit of marriage. Multiple studies from the 1970's through the present have shown that cohabitating couples are less stable than marriages. On average their relationships will not last as long and the level of satisfaction is lower.  There is also a higher incidence of domestic violence. Cohabitating couples also have significantly higher rates of divorce if they do get married.  The more partners someone cohabitates with before marriage, the less likely they are to have a stable marriage relationship.
In some countries with Laws based on English Common Law , couples who cohabit may find themselves legally married after a prescribed time. This is referred to as a common law marriage. 
- "Married men and women report very active sex lives—as do those who are cohabiting. But the married appear to be more satisfied with sex than others." 
- "... marriages that follow an extensive period of cohabitation ... end in divorce at disproportionately high rates" 
- common law marriage.