How long were you separated when you were with someone else? If it was just after separating, before the two of you had made your intentions totally clear, then I could see how it might be perceived as cheating. If on the other hand, you were separated with both of you planning on divorce as your next step, I wouldn't see how that would be cheating. At that point you would have both most likely been considering yourselves as "single", just without the piece of paper. If this was the case, I don't see it as cheating.