Random, I do understand what you're saying but it's not as 'black and white' as that, or at least I don't think it is.
If you've never been in the position of being truly tested - and I mean by this that you've met somebody and felt an incredible connection to them - and they with you - and have had to consider 'what if?' because the possibilities are there, then you will not understand what it's like.
It is a conscious decision, of course it is, but the steps to get there can be very blurred. There's always a period where one should pull back (yet there's nothing to pull back from, so no need) to a time where somebody is in too deep (by which time it's too late). Do you really think that somebody - anybody - embarking on an affair is thinking about the spouses/children in all of this maelstrom? The behaviour might well be cruel and underhand as you describe but in an affair, it is often considered only that IF there's discovery of the affair. I personally know several people who are maintaining almost double-lives and all the innocent parties are seemingly oblivious.
Good people have affairs. I know that is difficult to get your head around because it's awful behaviour but that doesn't change the essence of what a person's character is. I don't think it has much to do with 'morals' either as most people seem to be quite adept at deviating from customary 'good' behaviour (unrelated to affairs) as it suits them.
I agree with you that actively breaking marriage vows does mean that something has been 'damaged' in the marriage. I wouldn't necessarily say that it would be an easy decision to make, (which is what makes affairs potentially devastating to relationships) and I don't agree that having an affair for a subsequent time means that 'all barriers have gone'. What I think it does mean is that the barriers were not there in the first place, and that isn't an easy thing to stomach.
I can see that you're in pain Random and are clinging on to the hope that your ex treats the OW in the same way he treated you. He may not, you know. People don't enter into affairs lightly (one night stands and casual things excepted), they just don't. They risk losing all and are still prepared to lose it. Some people are with the wrong people to start and yes, in an ideal world people would end their marriages and relationships first before embarking on another one, but that's not how life always pans out. I believe that people don't leave relationships without consciously deciding to and, if they leave for another person, there's a chance that they will spend some time introspectively looking at their own contribution to the marriage/relationship they have left, and not repeat it with the next one. If you read the thread currently on AIBU, you will see that this is indeed the case, but it's hard reading if you've been on the receiving end.
No one person can hold another - marriage ceremony or not. I would think that any OW/OM would be more aware of that than most.