1
general::
Cohabit outside marriage, as in Bill and Anne lived in sin for years before they got married. This term, dating from the early 1800s, is mostly used in a jocular fashion today, when customs and views are more liberal in this regard. Also see LIVE TOGETHER.
American Heritage Idioms
2
general::
Phrase(s): live in sin
to live with and have sex with someone to whom one is not married. (Sometimes serious and sometimes jocular.) • Would you like to get married, or would you prefer that we live in sin for a few more years? • Let’s live in sin. There’s no risk of divorce.
McGrawhill's American Idioms And Phrasal Verbs