Yes, it is true that adultery is illegal in Massachusetts. Pursuant to Massachusetts General Laws, Chapter 272, Section 14 is it illegal for a married person to have sexual intercourse with someone other than their spouse. The penalty is up to three years in prison. The law continues to remain in effect. However, I have been told by Police Chiefs that they will never charge someone with the crime of adultery and in my lifetime, I have never seen it prosecuted.
There are many clients who want to file for divorce on the grounds of adultery. You can certainly do so but the court will not appreciate it and I would strongly discourage it. To file for divorce on the grounds (reason) of adultery you are required to list both your spouse and the person who he/she has engaged in the affair with. That person must also be listed as a Defendant and would also need to be served with the complaint for divorce. At the end of the day, filing for divorce based on adultery will not likely get you anything more and it will certainly manage to antagonize the situation where your spouse will want to fight over everything simply because you are making his/her life public and brining this other person into the divorce.
The only reason that the court would care about adultery is if a spouse was wasting marital assets by giving the other person extravagant gifts, vacations, etc.
I understand that your pride is hurt and you may never be able to forgive the betrayal but adultery is all but irrelevant when it comes to its criminality and any implications in a divorce situation. Instead I recommend that the person who was cheated on become a strong resistant person and make the best decision for their future rather than focusing on the injustice.
If the police and the courts do not care about adultery, then the rest of people must learn how to best benefit from the circumstances.