I wouldn't say that 'wise' people have affairs anymore than 'wise' people would walk across a busy road without looking. People often think that 'it will never happen to them' and perhaps people who have affairs fail to notice the signs at the start (when they would have control and could bring it to a halt), then because they've taken steps they shouldn't have - secure in the knowledge that 'it will never happen to them', they end up taking an irreparable step by which time it's too late as they're in too deep; enjoying the attention, still doggedly believing that 'getting caught will never happen to them'.
Many affairs are discovered, many are not - or not disclosed as discovered anyway.
I think arguing the point of whether affairs are 'mistakes' is semantics really; affairs are a by-product of lack of attention, craving something - newness or excitement or whatever.
I agree very much with Dahlen's affair-proofing post BUT in order to have that self-awareness, that drive to affair-proof yourself, you must have had experience somewhere along the line. In addition, people have different susceptibilities that could make them vulnerable.
As far as looking back on that time with 'fondness' goes, I can only relate to that in abstract. If you are in a relationship with somebody (clandestine or otherwise), you become close. Assuming that affairs are not just about sex (and I don't believe they are), you build a 'history' together, shared in-jokes, shared experiences, shared intimacies and talking. It isn't that difficult therefore to understand that when that relationship is suddenly ended, a person can experience grief and loss for it in the way that any other relationship would bring that about if the people concerned were close. In the same way that you can blot out the bad bits with the elapse of enough time, I'm sure you can look fondly at the bits that were good.
Whilst it may not be 'seemly' to grieve for a covert relationship or find happiness in remembering 'good times' in it, it would be foolish to dismiss feelings as invalid just because it's uncomfortable or acknowledge the basis for them.