Both your points above come down to the same argument, which is that if the married person wanted to stay faithful badly enough, then they would, regardless of any other party. It is not as black and white as that, so much more complicated, and WWIFN puts it better than I could ever: this is one of her posts from this thread: www.mumsnet.com/Talk/relationships/1074763-Triggers-for-emotional-detachment-WWIFN/AllOnOnePage (her husband cheated on her and she read loads about it and helped loads of other people in similar positions
I agree that Shirley Glass's book describes this very well, but I have seen this happening at all stages of the life cycle, not just in 30-somethings with young children.
To understand this in all its complexity, it is necessary to look at a person's attachment style (people with a dismissive attachment style are more prone to infidelity), their lifestyle vulnerabilities (if their family, friends and workplace are tolerant of infidelity and there are many opportunities), their selfishness and sense of entitlement and then finally, their state of mind and view of themselves prior to the affair. In this type of affair too, it is hugely significant about whether the affair partner pursued the attachment; who made the first move.
If we use a male example, what often happens is that a man is feeling low and even mildly depressed about life. Responsibility might be overwhelming and his partner's attention divided (young DCs). An older man might be feeling less needed by teenage DCs and his competent wife. He might be feeling old, grey and out of potential, having hit a ceiling at work. He cannot in truth say that his wife wouldn't understand how he is feeling, because normally she would give anything to help him with these feelings and support him, but he doesn't want to admit to his wounded pride and hates talking about feelings, anyway. So he hunkers down and suppresses how low he is feeling.
He would say if anyone asked at this point that he loved his wife enormously and was happily married. He would laugh at the notion that he would ever have an affair.
In a terrible coincidence of timing, along comes someone who wants an affair with him. It might not be obvious at first and he is normally in complete denial about his own intentions. He just knows that the positive mirroring and stroking he is getting from this woman makes him feel good about himself, when he has been feeling so bad.
He pretends that this is just a friendship and is a nice distraction in a life filled with mundane responsibilities. It makes the day go quicker, except that he now looks forward to their interactions more eagerly. He is becoming addicted to the feelings, but he doesn't know it.
Because he is by now in the grip of an addiction to the feelings, if not the affair partner herself, all he cares about is that it keeps coming in plentiful quantities. He might not even need the affair to involve sexual activity, but he does need sexual flattery and adoration.
At some (often subconscious) level, he realises where this could be heading, but because his lifelong beliefs are that people don't do this when they are happily married, he again subconsciously realises that he must detach somewhat from his wife to create a gap to let the affair partner in. He is not someone who can have a strong emotional connection to the woman he is deceiving, so it is necessary to reduce it. So he starts to create a distance and a detachment, from what was once a happy and fulfilling marriage, to a woman he has always adored. In effect, he needs to adore her less because if he adores her, he cannot get his fix from the addiction that has now taken hold.
I cannot stress enough how entirely subconscious this is, in most cases. And so often, it takes a long time for a man in this position to get himself to the position when he can say "yes" to an affair. Slow-burning work friendships, or any other friendship, provide the time needed to detach emotionally from a happy marriage.
Once the self-permission is given, the man will often minimise its impact and meaning; telling himself that this is just a bit of escapist fun, no-one need ever find out and get hurt, it will never lead anywhere, it will be a fun adventure (maybe the last) etc. The guilt often doesn't kick in until physical infidelity has occurred.
In the final analysis, this affair has happened not because of anything to do with the betrayed spouse, or the marriage. It is all to do with the unfaithful party; their state of mind, their character, their personality. They might never have gone looking for an affair and they certainly didn't want their marriage to end, or affected in any way in fact. But when an opportunity arose, they had to do a number on their marriage to allow them to say "yes" and they did this after they had long since become addicted to the positive mirroring and ego boost.
This person is often as shocked as their spouse when it all ends messily and horribly and the real consequences are felt. It is also a massive surprise when they are able to detach from the affair partner very easily, but of course it really wasn't about this person at all, but the feelings she evoked.
Does this help?