We are discussing adultery.
It may surprise many to learn that adultery is still illegal in 21 states (Washington is not one of them). The laws are slowing going by the wayside -- New Hampshire, for example, voted this month to repeal its adultery law -- but they still exist. And the penalties are nothing to sneeze at.
A quick Westlaw search shows that both your spouse and his or her paramour could be charged in Idaho. Each could be fined as little as $100 or as much as $1,000, and each could be incarcerated for a minimum of three months and a maximum of three years. Adultery is classified as a sex offense, but a conviction will not mean registering as a sex offender.
Please note that charges may or may not follow. And to answer the most pressing question: Most states that still have the law on the books enforce it rarely if at all.
One jurisdiction that does enforce the law is the military. The Uniform Code of Military Justice calls for as much as a year in jail and, at the maximum, a dishonorable discharge. According to the Associated Press, 30j percent of commanders who have been fired over the past eight years were fired for sexual misconduct, including adultery.
Imagine what havoc could be wrought in a child custody or support case if Washington's adultery law were still in place. First, there would the question of granting custody to a parent with a criminal record. Second, of course, there would be morality concerns. And, even if infidelity is not considered in a no-fault divorce, there may still be consequences when it comes to the best interests of the children.
While an unfaithful spouse may not be prosecuted in Washington anymore, there may still be consequences. Courts have been known to take bad behavior into account when deciding spousal support.
Source: USA Today, "New Hampshire Senate votes to repeal anti-adultery law," Jolie Lee, April 17, 2014