I do not think it is ever ok to cheat. It is being weak. And like someone said, its all a matter of perspective.
The neglectful partner could be exhausted from working 2 jobs to support a family. Sure, the stay at home partner might feel neglected and have a reason to feel so, but life is hard they need to toughen up. Even if the neglectful partner was neglectful intentionally or because of an addiction, the neglected partner should look at other alternatives other then cheating. Cheating is never ok. Thats why its called cheating.
Now another question is, if a partner that is neglected intentionally or due to an addiction should leave the relationship? That's a tougher question. And a harder line to draw. Because again, its a matter of perspective. I guess, its a personal decision that should have really been discussed between both partners before marriage. Probably none of us established clear cut boundaries as to when its ok to end a relationship.