Whenever there is a material change in the lives of the children, the court can reconsider a change in custody orders.  Historically, unmarried cohabitation has been frowned upon by judges as a moral breach.  A more conservative judge would modify custody to place the child with the other parent.  A less conservative judge will simply impose a restriction on the custodial parent that orders them not to allow any adults of the opposite sex to stay overnight during their periods of possession.  A liberal judge simply won't care. The bottom line is that it is a factor that can be considered to modify custody.  Before filing for modification, arrange for a consultation with a couple of different family law attorneys that practice in your jurisdiction.  They should be able to give you a better assessment of what types of judges will be handling your modification request.  From there, you'll have a better idea of whether you want to go forward with a motion to modify, or not.