I wouldn't torment yourself with this question op, his idea of love is very imature for a middle aged man and very rarely survives in the real world, I would say he is infatuated and on an extreme high with the flattery.
Two years they've known one another, sounds about right, their friendship has bled out onto their marriages, hers will be less so as her husband probably hasn't noticed as much, this is the crunch time.
This ow has become more demanding of his attention and he is complying, utterly heart breaking for you but this time is about you changing. Do your children know and your family, have you had any support, visited the GP with the actual details ? this infatuation needs bringing to light and you have currently given him numerous chances to haul it back and for it to remain private.
No pun intended but this is the climax, your displeasure and demanding loyalty is creating a deeper bond between them at the moment, you have become the enemy of their friendship and this is when emotional can fall into physical, if it hasn't already. Fuck, he's really put you through it and still he continues to gaslight you to keep his options and reputation in tact, that's a manipulative and scheming mind, he is not a kind man and is actually dangerous to your safety.
Please put yourself first, what you want is being stuffed down and you are accepting cruelty that you know you don't deserve, no one does in a marriage. I doubt whether this will last, she's younger, got young kids, has a husband who is probably younger than your h, your children will be disgusted and you are learning how to detatch, your husband is a fool and I'm sorry about that, but it in no way is a reflection on anything you have done, sometimes in life we meet, idiots, it's taken a while for your h to reveal how much of a monumental idiot he is.
It takes time to adjust your view of someone, especially your h when they have dissapointed you so much, so many marriages end over these EA's, whether couples stay together or not, I would say in a few years he will recognise how his marriage and family life have been ruined but he probably will not accept responsibility for that, he sounds the type to blame you.
I would lay money on this EA ending, but them remaining buddies into the future, as he will probably do this again with others and her, as your feelings turn to distain and disgust towards him, they eventually know the fuck up that's occured and he will search for that respectability, love and loyalty that he once had.
These men usually turn into aging middle aged creeps, alternating between pathetic sadness for themselves, humiliated father figures and angry little men who blame their wives for everthing in life.
The irony is, that if her husband ever finds out he will probably forgive her, betrayed women on the otherhand very rarely forgive and forget, such a fool he is.