If you're in Massachusetts the vast majority of divorces are "no fault". If one of you is claiming "fault" the only grounds are the following:

Mass. General Laws chapter 208, section 1: adultery, desertion, gross and confirmed habits of intoxication, cruel and abusive treatment, non- support, impotency, or a prison sentence of 5 or more years.

What did your lawyer say exactly about marital impropriety and how would you prove it?

Married 18, Together 20, Now Divorced
M: 48, W: 50, D: 18, S: 16, D: 12
Bomb Dropped (EA, D): 7/13/11
Start Reconcile: 8/15/11
Bomb Dropped (EA, D): 5/1/2014 (Divorced)
In a New Relationship: 3/2015