The United States is one of few industrialized countries to have laws criminalizing adultery. In the United States, laws vary from state to state. Up until the mid 20th century most U.S. states (especially Southern and Northeastern states) had laws against fornication, adultery or cohabitation. These laws have gradually been abolished or struck down by courts as unconstitutional. Pennsylvania abolished its fornication and adultery laws in 1973.
States which repealed their adultery laws in recent years include West Virginia in 2010, Colorado in 2013, New Hampshire in 2014, Massachusetts in 2018, and Utah in 2019.
As of 2019, adultery remains a criminal offense in 19 states, but prosecutions are rare. Although adultery laws are mostly found in the conservative states (especially Southern states), there are some notable exceptions such as New York. Idaho, Oklahoma, Michigan, and Wisconsin consider adultery a felony, while in the other states it is a misdemeanor. It is a Class B misdemeanor in New York and a Class I felony in Wisconsin. Penalties vary from a $10 fine (Maryland) to four years in prison (Michigan). In South Carolina, the fine for adultery is up to $500 and/or imprisonment for no more than one year (South Carolina code 16-15-60), and South Carolina divorce laws deny alimony to the adulterous spouse.
In the last conviction for adultery in Massachusetts in 1983, it was held that the statute was constitutional and that "no fundamental personal privacy right implicit in the concept of ordered liberty guaranteed by the United States Constitution bars the criminal prosecution of such persons [adulterers]." Massachusetts repealed its adultery law in 2018.
In Florida adultery ("Living in open adultery", Art 798.01) is illegal; while cohabitation of unmarried couples was decriminalized in 2016.
South Carolina's adultery law came into spotlight in 2009, when then governor Mark Sanford admitted to his extramarital affair. He was not prosecuted for it; it is not clear whether South Carolina could prosecute a crime that occurred in another jurisdiction (Argentina in this case); furthermore, under South Carolina law adultery involves either "the living together and carnal intercourse with each other" or, if those involved do not live together "habitual carnal intercourse with each other" which is more difficult to prove.
In Alabama "A person commits adultery when he engages in sexual intercourse with another person who is not his spouse and lives in cohabitation with that other person when he or that other person is married." Adultery is a Class B misdemeanor.
Since adultery is still a crime in Virginia, persons in divorce proceedings may use the Fifth Amendment. Any criminal convictions for adultery can determine alimony and asset distribution. In 2016 there was a bill in Virginia to decriminalize adultery and make it only a civil offense, but the Virginia Senate did not advance the bill.
In the U.S. military, adultery is a potential court-martial offense. The enforceability of adultery laws in the United States is unclear following Supreme Court decisions since 1965 relating to privacy and sexual intimacy of consenting adults. However, occasional prosecutions do occur.
Six U.S. states (Hawaii, North Carolina, Mississippi, New Mexico, South Dakota, and Utah) allow the possibility of the tort action of alienation of affections (brought by a deserted spouse against a third party alleged to be responsible for the failure of the marriage). In a highly publicized case in 2010, a woman in North Carolina won a $9 million suit against her husband's mistress.
