It's up to the OP what she does. If my husband were to have an affair, assuming that he hadn't been sloppy, careless and had taken precautions to protect my health, I wouldn't want to know. Perhaps it is one of those questions that needs to be asked upfront, along with "Do you want to be buried or cremated?"... never having to refer to it again.
If I decided to have an affair now, I would work very, very hard to make sure it wasn't discovered. I think it's discovery that is the hardest part; that and 'helpful' people giving you the news. If anybody has the right to impart the information it is the partner involved, nobody else's.
Pragmatism is not a vice and neither is discretion. Affairs really are happening in huge numbers and it is likely that one or more posters on this thread are currently blissfully unaware that their own partner is having an affair. It baffles me that some posters can be so obdurate and unable to accept other view points without insulting or posturing superiority over somebody else.
Nothing makes an affair 'right' because you can't change history. For that reason, I think that if somebody is committed to making their marriage work, it is a kindness to carry the burden themselves and ultimately forgive their own selfish actions in having the affair in the first place.
I think that people do not have a 'right' to information, to know every nuance and detail, not even in a relationship.
I also stand by the view that if you've never been 'tested' or in the position yourself, your view cannot be in any way balanced.