Adultery for residents of Illinois does not only cause a problem only with the adulterer's spouse, but also with the law as well. This is because the act of adultery (sexual infidelity to your legitimate spouse) is regarded as an offense by the state's criminal code. The law refers to both married people committing adultery, as well as to those who are non-married but have sexual intercourse with a married person.
According to the Criminal Code, Subdivision 25, Section 11-35 (formerly 720 ILCS 5/11-7), 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." The latter means that the act is known by the public (by the adulterer's spouse or anyone else) and is at odds with the community's accepted moral values.
As Section 11-35 continues, a married person is guilty of adultery when he willingly accepts sexual intercourse with someone other than his spouse. Therefore, cases of forced intercourse (rape) are not regarded as acts of adultery. It also is the most easy case of adultery to prove, as no sane person can forget his marital status at any point.
An unmarried person is liable for prosecution if "he knows that the other person involved in such intercourse is married." Therefore, it is a defense to prove that you -- the unmarried defendant -- were not aware in any way (for example, by spotting an engagement ring, observing family photographs or being warned by your married partner or a third party) about the other person's marital status.
Adultery is a Class A misdemeanor in the state of Illinois. According to Section 5-4.5-55 of the Illinois Unified Code of Corrections (730 ILCS 5/5-4.5-55), a Class A Misdemeanor is punishable by a fine not exceeding $2,500 for each offense. Furthermore, the fine may be imposed in addition to a sentence of conditional discharge, probation, periodic imprisonment or imprisonment not exceeding one year.