I mentioned somewhere here that I consider putting such expectations on someone unethical. If my partner has an affair they would tell me and I would be happy for her or neutral, but if they were hiding it, and lying then there was no really a relationship to begin with, just a lie that should end sooner than later.

And in another thread, I said that the social punishment for people who cheat is too severe, often being social isolation. For an infraction I would personally consider as bad as cheating on a diet. For something that causes no harm to anyone.