Massachusetts is one of five states that -- at least on paper -- consider adultery a felony. The others are Idaho, Michigan, Oklahoma and Wisconsin. It is a misdemeanor in about half the country.
Here, the law is Chapter 272: Section 14, and reads:
A married person who has sexual intercourse with a person not his spouse or an unmarried person who has sexual intercourse with a married person shall be guilty of adultery and shall be punished by imprisonment in the state prison for not more than three years or in jail for not more than two years or by a fine of not more than five hundred dollars.
In practice though, it is hard to find any enforcement. Adultery as with marriage is a civil matter. However, in 1980, police discovered a couple obviously enjoying each others' company in public in a van. They admitted that they were married to other people. He paid a $50 fine, but she fought the charge. She claimed she had a right to privacy, citing abortion and contraception case law. The Supreme Judicial Court figuratively snorted and said those had nothing to do with each other. She lost and 272:14 remains the law of the commonwealth.
Commonwealth v. Stowell, 389 Mass. 171, 449 NE2d 357 (1983).