Cohabitation is absolutely not acceptable in the Church of Jesus Christ of Latter-day Saints, cohabitation is grounds for disfellowship and/or excommunication if the presiding authority deems it appropriate.  
 
Cohabitation is a violation of the Church's standards for the Law of Chastity, there is too much temptation and potential for things to evolve into fornication in those situations so the Church 100% prohibits cohabitation between a man and woman who are not married to each other.