I think the wording isn't quite right Miss Chief, accceptable or excusable I'm not sure I can get on board with. 'Understandable' however, then yes, I think there's certainly a case for that because there can be variables.
Examples.
Married couple, one has altzimers (sp) and can't remember the other partner let alone be a proper companion so they meet and have a relationship with another. They're still married but it's cheating and to my mind perfectly understandable.
A couple going through a divorce but not yet completed but living seperate lives. One of them meets someone else during this time and a relationship ensues. The married couple start to resolve differences, the other relationship is discontinued and get back together. They were still married so effectively cheating but still understandable. (This one I can speak from experience on having technically been the 'other woman').
A relationship where one member has lost their sex drive completely and allows the other to fufill their needs elsewhere. Again, it's still cheating but by mutual consent and understanding.
These are a few examples I can think of on the spot but I'm sure others could think of more. Is it acceptable or excusable to get your end away for a night because you think you won't get caught or there's problems at home and you want some relief? Absolutely not. But it's not always black and white either and before a judgement is made facts need to be established.
Relationships are always very subjective to the people involved and while I think a good friend is always a sounding board and can offer advice or an opinion if asked, I think sweeping generalisations are better avoided. No one ever really knows for sure what goes on behind closed doors.