Earlier in my own recovery trying to put people in boxes was part of my coping skills. The "why" around the cheating and/or alcohol use/abuse was important to me.
As time has passed what is more important is:
1. Is either situation/both something the other person wants to appropriately acknowledge and address? What does it look like when they are "working" on his/her stuff? Do I believe it is genuine?
2. Is the infraction something I can live with? Am I willing to do my own work around this while maintaining an appropriate delineation of what is mine to work on and what is NOT (another's addiction is not mine).
In my instance more was revealed that made the "why" of it less important. I suspect this usually happens.