Not necessarily true. Depends on the state. In fact, in Idaho, Massachusetts, Michigan, Oklahoma and Wisconsin, among others, it is a felony crime punishable by prison.
"Adultery laws, which make sexual acts illegal if at least one of the parties is married to someone else: Alabama, Arizona, Florida, Georgia, Idaho, Illinois, Kansas, Massachusetts, Michigan, Minnesota, Mississippi, New York, North Dakota, Oklahoma, South Carolina, Utah, Virginia and Wisconsin."
https://www.salon.com/2019/05/06/adultery-and-fornication-why-are-states-rushing-to-get-these-outdated-laws-off-the-books/