As yesterday's post highlighted, marital infidelity is common.
In the past, however, every state had a law outlawing adultery in some way. Before all states instituted no fault divorce in the 1970s, adultery was often the only grounds for a divorce, and each state had a law outlining adultery as cause for a legal divorce.
The most recent comprehensive listing of adultery laws I could find come from A Guide to America's Sex Laws published in 1996. While apparently 23 states still have adultery laws on their books, most legal scholars believe the US Supreme Court's 2003 Lawrence v. Texas decision upholding a constitutional right to private, consensual sex pretty much invalidates the remaining adultery laws.
By 1996 most of the Western states had eliminated their adultery statutes. A number of others retained adultery only as a legal grounds for divorce or grounds for barring an adulterous spouse from legal right to property.
For most states criminalizing adultery, the statute defined the crime as a misdemeanor. Five states, however, defined adultery as a felony in 1996: Idaho, Wisconsin, Oklahoma, Michigan, and Massachusetts. Three Deep South states -Mississippi, Alabama, and South Carolina- have statutes which appear to make adultery a crime only if the adulterous pair are actually cohabitating. The laws from Massachusetts and Michigan mirror each other including a caveat that if a couple divorces but then live together, then they can be charged with adultery.