Solost Okay, you tell me whether you think this is what happened?
You had a great marriage and loved eachother deeply. But you never had a discussion throughout that marriage about how you would both deal with temptation. If you ever discussed infidelity, you were both utterly condemning of it and assumed that if someone was seriously tempted by someone else, something would be seriously wrong with the marriage. Therefore you never once acknowledged that temptation is in fact perfectly normal and that in a long marriage especially, the lure of sex and romance with someone new could become pretty intoxicating and hard to resist.
So your H, who always took such a hard line about affairs and temptation, was completely blind-sided when he found himself tempted by an affair. He never thought this could happen to him and consequently, placed far too much emphasis on the heady feelings of excitement and lust and convinced himself that since he wasn't the "sort of man" who had tawdry affairs, this must therefore be love.
In other words, he is a romantic idiot who cannot tell the difference between limerance and proper love. In his idiocy, he continues to believe that it was nobler to leave a good marriage and his family, once he'd been unfaithful. No doubt he is now surveying the utter folly of that decision, the "love" feelings are fading and replaced by rows and recriminations and a huge part of him is realising that he has made the most monumental mistake.
However, he clings to a mixture of pride/inability to admit a mistake and feelings that he has made his bed and should "suffer the consequences".
This is man who cannot bear to be wrong and likes to pretend to himself that he is a "good man" who cannot live with deceit, so he simply had to "do the right thing" and leave his wife and children for what was actually an infatuation.
Damned right he is telling you the truth when he says he still loves you, there was nothing wrong with your marriage and that you were perfect for him. Even he realises that he cannot rewrite history, but of course this adds to his confusion, because he never gave any houseroom to the notion that he was just like every other human being and capable of temptation. He thought he was above all that, but it turns out he wasn't and he was like so many other people in long marriages and let his dick and his need for romantic escapism, overrule common sense and pragmatism.
Romantic idiots like this shouldn't be let out on the streets after dark, because they cause untold harm and destruction to others' lives and especially their own. He isn't the first and he won't be the last person to confuse limerance with love and unfortunately, more men are doing this than ever before.
The root cause of it is a complete lack of realism about temptation.
Now, I suspect what's happening here is that because of his romantic idiocy, he is utterly bewildered about how he's come to be in this position. He is terrified that he has burnt his boats with you and no doubt did a good job of "re-claiming you" with sex when he got kicked out and stayed the night. He might even have engineered the row with the OW, so she would do just that and provide him with an excuse to have sex with you again.
He wants to keep you "on side" so that when this does finally burn out (and by the sounds of it, it will) he can be welcomed with open arms back to the happy homestead, contrite, but not a great deal wiser for his "adventure".
Now if you know that you can try to forgive your particular romantic idiot, then kick- start the plan I suggested and have some fun doing it. Note I said "try to forgive" because in truth, you cannot know yet whether you ever will forgive - and when your plan works, you will need to completely change the conditions of your future relationship and knock his ludicrous delusions about being a "decent man" into shape. He is going to have to change a lot but so are you.
Once he sees you moving on (and use that jealousy of you meeting other men to spectacular effect incidentally) and decides he wants to come back, say no, until he has lived on his own having ended his relationship. Don't let him back too easily. Make him fight for you. Who knows, by that time you will have lost patience and would prefer a more realistic man who doesn't get carried away by the lure of a romantic adventure?
But get that plan into action now and you will see truly miraculous developments.