IDIOMS

live in sin

(intransitive, idiomatic) To cohabit as if man and wife without being married.

Example: 1922, John Galsworthy, A Family Man, act 2:
  Ralph: Athene gone and got married?
  Builder: No. It's—it's that she's gone and—and not got married. . . . I suppose you'd have me eat humble pie and tell Athene she can go on living in sin and offending society.