You raise an important moral question, but it is also a boundary issue. In most instances, you have no way of knowing what will result from your revelation. In the long run, perhaps the cheater's spouse and children will benefit from what you choose to disclose--but it may also cause unnecessary harm and pain at the worst possible time. In either case, you are choosing to intervene in a relationship that you are not party to--and for which you have no responsibility (that is where the boundary issue comes up). If you opt to toss a grenade into this family unit, you will walk away feeling self-satisfied (after all, the cheating bastard deserves it!) but you have no idea what the consequences will be for the rest of the guy's family. Is that really your place? Sometimes not stepping into a situation and trying to rescue the innocent or deliver justice is the hardest thing to do ... but it may also be the ethical thing to do.