Adultery is one of the most common reasons couples seek divorce . Most states define adultery as "sexual intercourse between a married person and someone other than their spouse", however this definition can vary from state to state. In North Carolina for example, adultery is defined as any sexual relations between a man and a woman who are not married and cohabitating together, whether legally married to someone else or not.
This information is from My Family Law:
http://www.myfamilylaw.com/library/Sex-Offence s/Adultery