Legal Question in Family Law in Utah

If you and your ex were awarded joint legal custody with a Decree that incorporates a Parenting Plan, then the terms of that Parenting Plan are binding. The type of restriction you mention is actually fairly common and enforceable. If the relationship you're in with your girlfriend has been fairly long term, you can file a petition to modify the Decree and the court may ease or lift the restriction.

Why would you want to introduce your children to a temporary girlfriend that sleeps over? It just adds confusion and degrades the value of marriage and moral values. That behavior says that sex - which leads to children - is not to be valued and children can be placed into unpleasant circumstances over which they have no control. Girlfriends come and go. Parents are supposed to be permanent. Your behavior confuses these roles.