I'm going to give you my thoughts in response to your questions, but I'm going to pose several others. I suppose I ought to forewarn you too that a lot of these questions might challenge the script you're currently carting around and the one your H is wanting you to believe and might even believe himself. Some people don't want that level of challenge though, so I will understand if you don't want to persevere with this.
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How could he?
- Because by that time, he had already long since given himself permission to have an affair. The die had been well and truly cast. It wouldn't have seemed such a leap, because of the 100 steps that preceded it, each one serving to normalise the last. At this stage, he was undoubtedly well and truly in the grip of the insanity that is an affair and a more relevant question at this point would be "why wouldn't he?".
By this time, he would have moved away from any "why would I?" questions (if indeed he ever asked these of himself) and into "why not?" - so the justifications would have been left long behind and moved onto potential consequences. I'd wager that those consequences were some or all of the following: You would never find out, especially as you were out of the country and what you didn't know, couldn't hurt you. This will burn brightly for a while and then fizzle out, because it's all about intense physical chemistry and there aren't any strong feelings involved. He might not get another opportunity like this again. He thought he would be able to compartmentalise successfully, keep a secret without harm being caused to either of you and once this had ended, your marriage and him and you as individuals would be still completely unscathed.
- How could he lie in the face of incontrovertible evidence?
Firstly, because of fear and panic, but more meaningfully, because of his basic character and habit of lying. He had been coping with the guilt possibly even better than he thought he might and this was easier when he didn't have to see you all the time when you were away. I expect he wondered whether you would possess magic powers to instinctively know what he had been up to as soon as you locked eyes - and wondered whether it would be best to get a confession in first....but instead he watched a programme that told him what he wanted to hear and comforted himself that telling you would only hurt you unnecessarily and that he should bear the (previously non-existent or at best, lightly held) "burden of guilt" as his "penance".
Of course, the truth of this is that he didn't regret the affair and probably didn't feel much guilt - at this point. A large part of the "guilt" you are seeing now is because he has been caught and he is now having to face the consequences. A lot of those tears are in fact self-pity at the chaos and ruination to his own life, not yours or his son's.
And the real reason he didn't tell you was because he didn't want to live with the consequences of you knowing. Keeping secrets from a parter are often dressed up as being for his/her benefit and while some of that might be true, it's never the whole story and the person most likely to benefit from the secret was him, not you.
Which leads me to some observations and questions.
I don't doubt for a second that he loves you deeply. I don't doubt for a second that whatever bargains you are trying to make about how if your relationship has been better, this wouldn't have happened, this has less to do with your relationship and far more to do with him.
Affairs can happen in good marriages, if you accept as I do that no marriage is permanently characterised by hearts and flowers romance, amazing frequent sex all of the time and exclusive, focused mutual attention and adoration. That in every healthy normal marriage, there will be times when the focus shifts and when life gets in the way for a time. It is too easy and simplistic to put the spotlight on an infidelity-blighted marriage and find causative factors, because those would be found (and far worse) in most marriages, for some of the time.
So while affairs can happen if there are no relational factors at all, they never happen if the individuals have a mature and pragmatic approach to infidelity, lots of self-awareness and a core belief that it is never justifiable to deceive and lie.
It's mature and pragmatic to acknowledge that everyone is potentially vulnerable to the attentions of someone else. It is disingenuous to say it doesn't feel good when someone else desires us, respects us or adores us. This can happen without reference to our primary relationships, but the wise move is to acknowledge it and swerve the tempation, not feed it until it gets too difficult to resist.
The self-awareness comes from knowing your boundaries and your weak spots. Many men in your H's position are suckers for damsels in distress and their chosen affair partner often turns out to be volatile, needy and with a train-wreck back history of failed or abusive relationships. Hence, the weak-spot is rescuing tendencies which are all the more emphasised if his wife is competent, level-headed and independent; the antithesis of the OW.
Other bits of self-awareness are some acknowledgement that a person is at a vulnerable time in life (i.e. mid-life) and needing an escape, feeling that the adventures in life might be over, feeling depressed (depressed people are especially vulnerable to affairs), recognition that one's own esteem has in the past been defined by one's attractiveness to others - and so it goes on.
The last bit is about core beliefs and these would have taken however many years your H has lived, to be shaped and formed. What he needs to concentrate on is what his beliefs were about fidelity before he started this affair - not what he believes now.
The key to this all is personal responsibility (hence do please jettison the idea that the OW jumped him) and what behaviours and character traits existed in your H before this happened. Focus on him as an individual, then have a look at what your lifestyle vulnerabilities were, but recognise that making changes to your relationship is the easy bit. Changing a person's character, belief and behaviours is much harder, but it really is the key.
Good people who love their partners are having affairs, but they only do so if they under-estimate their vulnerability to temptation, fail to take precautions based on self-awareness and are then able to lie and deceive, if only for a short time.
Because I suspect you are focusing on the start of the affair, what I want you to do is to go back long before it and timeline what led up to it. You will have done this in relation to your relationship already, but I want your H to focus instead on himself and what caused him to detach and under-invest in your marriage, because affairs can only happen if there has been detachment and under-investment.
Sometimes those two have occurred long before the affair and sometimes they happen, because of the presence of an affair opportunity.
Timeline the lot and it is essential that you know when your H had first contact with this woman. Start the affair clock from this point and no later, to assess when the under-investment and detachment preceded that date, or followed it.
Whatever you do, don't fall into the trap of thinking that because this affair was "just" a sexual fling that wouldn't have happened at all if the OW hadn't offered, at a time when your relationship needed some attention, that is the whole story and that all it will take is an improved relationship and you getting over your hurt, to out this behind you.
Affairs mean something. But truly, they mean more about the person having them than anyone or anything, else.