Short answer, nothing that selfishly hurts another is right. Good people do bad things sometimes...it doesn't make them bad people, just human.

But whether a hurtful act, something that is fundamentally a betrayal of trust ("cheat" impliles sneaking....open relationships are an entirely different subject), can be justified is a harder question. "Justifyable" is a tricky word. People naturally come up with justifications for what they want to do. The difference between a "reason" and an "excuse" pretty much just depends on whether you're speaking or listening, as far as I can tell (and it's a puzzle that my brain's been working on for ages). But I think that the justifications that are classically offered for cheating are irrelevant. The act shows a lack of respect for the person being cheated on....whether they ever learn of it or not. Why it was done....well, it still reveals a flaw in the relationship. In, as always, my humble opinion.