Oh Karen, I'm so sorry you are going through this. My sister went through this, over many years. They eventually split after almost 30 years of marriage, because she had always said she would throw him out when she caught him fooling around. He had fooled around for years and finally got careless. I suspect he wanted to be caught.
A few thoughts (and please, everybody, don't misunderstand me, please read on before you bite my head off) -
I like to think about this from the bloke point of view. This is not justifying, it's merely trying to get into his head. It makes it easier to get a feel for what to do to anticipate his next move, or next statement.
To a lot of men, sex is not love. It's just sex. And that is what was happening with this woman - you were a bit too busy, he probably didn't want to put sexual demands on you, you were a bit far away and distracted, he'd simply get sex elsewhere so he didn't miss out in the meantime. It's a bloke thing, especially common if the bloke is self-centred and/or weak.
The lovey-dovey stuff, the cuddles, the affection, the "I will telephone you to find out how you day is" - that is him loving you, wanting to stay married to you, caring about you as deeply as he can care about anyone other than himself. That is what he sees as loyalty. For you to not be as welcoming with this - he simply can't understand it, he never intended to leave you or be unfaithful - in his eyes he was NOT unfaithful, he was just ensuring his sexual needs were met without pressuring you. In his mind, he was being considerate of you.
Now let's think about the other woman. She was at this event with her mother - what sort of mother will allow her daughter to hook up with a married man? What sort of woman WILL hook up with a married man?
So we ask - did she not know he was married? Or did she know, but did he give her some reason to hope there could be a future with him? He says HE ended it, but it sounds to me like SHE did. "She didn't suit me" is his way of saving face, of dealing with her rejection. "She didn't suit me because she shouted at me when she found out I was married, she accused me of some awful things that are simply not true, such as being deceptive - I'm not - of being unfaithful - I'm not, I've never loved another woman like I love you, I've only had SEX with other women, frequently, when my needs get out of control. Who needs a nagging mistress?"
And what way is it to treat ANY woman, to simply use her for sexual gratification and then to dump her, totally ignoring her feelings? He clearly has no idea how women think or feel. Not only did he treat this other woman shamefully, his apparent inability to comprehend how much this upsets you shows hoe incredibly self-centred and weak he must be. The measure of how he treated this other woman IS unfortunately also a measure of how he treats you.
That said, there is still hope for your marriage. It depends on a number of things:
What do you want out of marriage? What do you want out of THIS marriage? What have you got out of this marriage? What does this marriage give you that you value, that is irreplaceable? What do you need? Does he give it to you? Is he capable of giving it to you?
Now again - what do you value about yourself? What are you capable of as an individual, not dependent on any other emotionally, financially, physically? Is this enough for you, or do you need more, in the form of your husband, to feel fulfilled in life? Is so, why? if not, why not?
I'm not asking for you to give me your answers, only to give them to yourself. Be aware, he is always going to think the way he does, act the way he does. He is weak, he keeps doing this, he is already rationalising why he broke up with her. Denial is not just a river in Egypt, it's his way of life. Don't make it yours. You need to choose if you can live with this, and if so, how you will deal with it.
But remember, you do have a choice. Stay with him, or kick him out. Either choice will be the right choice, if it is made with a clear head. Whichever you choose, you must live with it and own it. No regrets. Move on with whatever comes now. Even if your choice is to simply wait and see, this is a point from which to move forward.
But whatever you do, don't stagnate. Don't just let things lie and let him think everything peachy keen if it's not; similarly, don't bet him up over it but make HIM move forward too.
Good luck, whatever you choose to do.
Marg