Their definition of adultery is rarely clearly explained. Maybe the reason for that issue to their flip flopping on the issue.
I understand it currently means the same as pornea with the only difference being marriage.
Pornea involves the manipulation of sexual organs (but not breasts, no doubt the divorce rate would be even greater if it included this).
At one stage adultery didn't include sex with a person of the same sex or an animal.
This is a suspicion but I bet it does not include child sexual abuse. One of the RC case studies helps to confirm this.