The problem with this thread and all those where there are mainly black and white opinions, is that the views are often limited to the posters' experiences - whether they have had an affair, been betrayed, or have observed other people's affairs. It's rare to have impartial and unemotional responses that allow for shades of grey :)
The problem with this is that these opinions and experiences are written as if they are facts and apply to everyone in the same situation.
For example, it's not true that all affairs are discovered. In RL I know of some which have never been ( and no, I don't mean me before anyone jumps in with that accusation.)
Similarly I've known of couples where affairs have been openly conducted and tolerated, because they have a different expectation of marriage.
There are hundreds of examples of well known people- over the centuries- who have openly had affairs and their spouses have accepted this or turned a blind eye.
I'm not saying this is right or wrong- but what I am saying is that no matter what you post here it is YOUR opinion only.
On balance, my opinion is that most affairs cause hurt to one or more people - but at the same time I accept that some couples can accept affairs and they aren't such a big deal to them as they are to other people.