Adultery is still a crime in Massachusetts. The criminal provisions of the commonwealth's marriage law doesn't cut divorced partners any slack.
Chapter 208: Section 40 Cohabitation after divorce
Persons divorced from each other cohabiting as husband and wife or living together in the same house shall be held to be guilty of adultery.
And the penalty for such criminal activity (although rarely brought to bear) is:
Chapter 272: Section 14 Adultery
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.