What you are mentioning in the first para and I think Reha also pointed this out is not faking, its just cheating.. and whoever is doing it is actually committing a crime. Wutever is wrong, I cannot even argue on that.
My point always is that why cannot be a girl/guy can be so much into each other so that they can just let the other person know what is actually happening, even if the guy is checking-out a gul.. his wife/gf should be able to understand that if the guy is telling her that, atleast he is not hiding anything which is so much better than all sorta cheating or faking..