Actually, I believe it is immoral to keep extra-marital affairs secret from ones spouse, but not immoral to have an extra-marital affair. If this offends someone, then it says more about the person who was offended by it, than it does about the people who participated in the affair.
What does it say about them?
That they think about sex and marriage in a very regressive and traditionalist manner, and they believe that they have a right to control their spouse as if their spouse was property and not another person.
I mean, I'm just a little bit confused with this view. So let's take a hypothetical marriage between a man and a woman. The guy goes out, sleeps with a co-worker, goes home, tells the wife, the wife gets very upset. Is the wife being unreasonable here? From what you've said, you don't think what the man did was immoral, since he told her.
Or are you talking about a situation where both people agree that they can sleep with other people, so long as they are told about it
(and presumably asked beforehand, which I think is common in open relationships). If this is what you mean, then I agree with you. I would never agree to an open relationship myself, but I have no issue with other people doing it so long as it works for them.
But if you're saying that in a regular marriage, if someone has an affair and tells the other about it, that it's unreasonable to get upset with that person because traditionalist / etc, then that is patently absurd.