Review your decree. Does it say the maintenance terminates on certain events? Does it say the maintenance is modifiable? Unless there is language specifically saying it terminates upon cohabitation, it does not so terminate. If modification is not barred by the decree (which can only be ordered if you and your ex agreed), then co-habitation might be a basis for modifying the maintenance. You should talk to a family law lawyer in person, with your documents, so you can get a full answer after all relevant facts are known.