Adultery isn't just a crime in the eyes of your spouse. In 21 states, cheating in a marriage is against the law, punishable by a fine or even jail time.
The New Hampshire legislature is voting on a law Thursday which would repeal its anti-adultery law. Currently, adultery is a Class B misdemeanor and punishable by a fine up to $1,200 in the state.
"I don't think there's any appetite in New Hampshire to use police powers to enforce a marriage," said New Hampshire state Rep. Tim O'Flaherty, the bill's sponsor, during a public hearing last month.
Last year, Colorado repealed its anti-adultery law.
And states' anti-adultery laws are rarely enforced, a vestige of our country's Puritanical beginnings, said Naomi Cahn, a law professor at the George Washington University Law School.
"I suspect it's not something most people having non-marital relationships are thinking about," Cahn told USA TODAY Network.
States with anti-cheating laws generally define adultery as a married person having sexual intercourse with someone other than their spouse. In North Carolina, adultery is when two people "lewdly and lasciviously associate." In South Carolina, adultery includes having "habitual carnal intercourse" with a person who is married to someone else.
In practical terms, committing adultery poses very little threat of prosecution, but it could have civil consequences, such as impacting custody battles during a divorce, said Melissa Murray, law professor at the University of California - Berkeley.
"There's a stigma attached to adultery," Murray told USA TODAY Network. "The fact it is a crime maintains that stigma."
Cheating on your spouse can even be grounds for losing your job. This is particularly true in the military, where adultery has a maximum punishment of a dishonorable discharge and confinement for one year, according to the U.S. Uniform Code of Military Justice. In the past eight years, 30% of the commanders fired lost their jobs due to sexual misconduct, including adultery, the Associated Press reports.
Although adultery is a misdemeanor in most states, some -- including Michigan and Wisconsin -- categorize the offense as a felony.
Punishments vary widely state-by-state. In Maryland, the penalty is a paltry $10 fine. But in Massachusetts, an adulterer could face up to three years in jail.
In 2003, the Supreme Court ruled in Lawrence v. Texas that sexual activity between consenting adults is legal, and state laws banning homosexual sodomy are unconstitutional.
Whether the Lawrence case applies to anti-adultery laws remains "an open question," Murray says, and the continued existence of these laws "raise very important privacy issues."