Proud - no, I'm not saying that it will never work out if one partner has had an affair & that it is always better to end it. There are a couple of MNers who have worked through it and seem happy enough. I have friends who have worked through it and are still together, however, not one of my friends thinks that what they have now was worth the pain they went through to get it and would not have opted to work through it, if they had known how fucking awful it would be and how different their 'new' relationship would be to what they thought it would be.
Seldom does it work and the pain you put yourself through is immense. You don't end up with what you had (which might not always be a bad thing, but in most cases is not a good thing).
The key thing is that the person who had the affair has to be the proactive one, they have to be offering solutions, they have to be doing the soul searching as to how they could do that to the person they love, they have to be the one reading books/getting counselling. They have to be the one who is deeply apologetic AND they have to take 100% of the blame. They have to understand (for themselves!!) that no matter what was wrong in the marriage/their own lives that enabled them to do this that their responsibility was to discuss this with their wife/husband and sort it out - there isn't anything that 'lets them off the hook' and it is RARE that someone who has had an affair thinks like this.
If you do take them back, for most of them, it's just the green light to keep treating you/the relationship like shit and for many/most seems to give them 'permission' (in their own warped minds) to do this again.
It is fucking hard to walk away from a relationship when you still love the other person, it is hard not to give them 'one more chance' to prove themselves to be the person you thought they were, it's hard to 'make yourself single' when every fibre of you wants to be with them, to fix it - but it's a damn sight harder to put yourself through being the 'wronged' party trying to fix it :( Of course most of them will agree to 'never do it again', claim to be 'sorry', claim to 'regret it', cry, beg forgiveness etc - don't be fooled - that's the easy part... as the 'wronged' party you have to walk away, you have to make them come to you, you have to make them beg YOU for another chance and they have to prove themselves off their own back, without you telling them what they need to do. THEY have to fight for their marriage, not the wronged party.