720 ILCS 5/11-35: Adultery.

(a) A person commits adultery when he or she has sexual intercourse with another not his or her spouse, if the behavior is open and notorious, and:

(1) the person is married and knows the other person involved in such intercourse is not his spouse; or

(2) the person is not married and knows that the other person involved in such intercourse is married.

A person shall be exempt from prosecution under this Section if his liability is based solely on evidence he has given in order to comply with the requirements of Section 4-1.7 of the Illinois Public Aid Code.

(b) Sentence.

Adultery is a Class A misdemeanor.