Adultery is defined as consensual sexual relations when one of the participants is legally married to another. Some states have laws making it a crime and and in many states it is grounds for divorce for the spouse of the married adulterer. Adultery statutes are based upon the married status of the parties, and a breakdown in the marital relationship is not a defense. Adultery is rarely prosecuted, and in no-fault divorce (or dissolution) states, adultery is legally not relevant. In some states, adultery statutes require heterosexual intercourse to have taken place.
In states where adultery is a crime, it is a defense if the person accused reasonably believes that he and the other person are unmarried persons. The burden of raising this issue is on the defendant, but this does not change the burden of proof.
The following is an example of an Arizona adultery statute: