Humans have decided that trust is how we operate. It is the basis of contract law, of commerce, of nations, of democracy and last but certainly not least, it is the basis of personal relationships. If we are willing to try people for breaches of trust in these areas, why then when someone cheats on their partner can the partner not sue for breach of trust? That's not even accounting for the emotional trauma. I don't think that people should go to prison for it but awarding some level of damages to the other person seems fair to me. I feel like I should mention I've never cheated or been cheated on, this is just something I was thinking of earlier today. Edit My view has been changed. Whilst morally speaking I think there should be some method of remedy for the person being cheated on, the practical impossibilities regarding proof make it too hard an issue to solve legally.
Infidelity should be punishable under civil law.