It's early days for you and I understand your confusion about the advice you may have been given.
One of the reasons I think you'll find the book helpful is because its general philosophy accords with yours. That good people have affairs and that it doesn't have to define someone as a "bad" person, as long as that person learns from the experience, works on the various vulnerabilities and commits never to hurt or deceive people again.
As a general principle, I agree that affairs are about expressing a yearning for something, but that either the yearned-for thing cannot be met in the marriage (secret sex with someone new) or cannot be expressed within the marriage (feelings of dissatisfaction, temptations, the need for a separate adventure).
The couple who can talk about the challenges of monogamy and admit their difficulties (if any) and can take risks about disclosing the real extent of their dissatisfactions, crushes that might occur or their need for unthreatening time out from one another, tend to protect their marriage from this sort of crisis.
I can see why it seems obvious to you both that your H didn't feel his needs for affection and sex were being met, he expressed them and then gave up and found that elsewhere, are the reasons for this affair. However, I suspect it is rather more complex than that, evidenced in part from what you say in your last post.
You say that you thought it more likely that you would be unfaithful of the two of you. I want you to think why that was - and what needs you felt were being unmet for you that drove you to that conclusion? One of the best things you can do as a couple is to talk about how both your needs are going to met in this new relationship. I'd hazard a guess that you will find that your needs were being unmet hugely and for some time.
The really important question I want you to consider too is that although I understand your rationale that your H took affection from the first person to offer it and that he wouldn't have done so, had he been getting that need met at home is this:
If getting affection and love from you was what he wanted, why did he not come to you and tell you about the OW before things went too far?
Now, that might seem an obvious and silly question, because of course the reason is that he wanted to go ahead with this exciting new adventure and telling you would have prevented that, but your P needs to be searingly honest with himself - and you - about whether he would have accepted the opportunity to have this adventure, regardless of feeling undesired.
That can be a difficult hypothesis, because he cannot know this with certainty unless he can think of times when he had a similar opportunity, but turned it down because he had no need of it.
I'm guessing that your P has been having some mid-life reckonings and feelings that all the adventures in life were over. Then along came the chance to feel alive and young again and since in many 23-year relationships, people's partners are not on their best behaviour and gushing about how desirable one is, the effect of hearing all this from someone new and attractive was like a bomb detonating.
This might have had the same effect even if your relationship was sexual and nurturing, because you had become familiar and comfortable.
Now, I'm sure that the lack of sex and affection were easy justifications for your H to make to himself at the time and are typical of a basically decent person who needs to find a valid reason for doing something he believed to be fundamentally wrong, but the strange thing is that people are having similar affairs, even when their primary relationship is sexual and affectionate. The common demoninators however are the length of the primary relationship, the age of the unfaithful party, mid-life reckonings and a feeling of being old and out of potential.
I want you to think about what that suggests, have a read of the excellent book and learn from what she says about taking a holistic approach to infidelity, because although evidently there were relational problems in your case, it is likely that there were also some individual vulnerabilities and given your P's job and travel, some lifestyle vulnerabilities too.
If you both want to prevent this happening again (and not just by the absence of opportunity) it makes sense to look at the whole picture - and not just the failings in your former relationship.