I think what the OP's H did was a sad error of judgement. And you have every right to feel betrayed SBB.
My marriage broke down horrendously over some crap issues, but sex had been a major problem for a very long time. I have quite a high sex drive and it is horrible. Really horrible. Not just the physical ache of wanting the sexual urge to be satisfied, but the sense of frustration at it being there all the time.
It is not a choice, but I do have to work very hard at switching it off. I don't know what I would have done if the opportunity to just get rid of the bloody feeling had been an 'easy' option.
It is like hunger for food, it can get to a point where in the end you would eat stuff just for the sake of making the gnawing ache go away. It can become very consuming.
I didn't go anywhere else, I am 'quiet', loyal, faithful, thought I had high morals and have gone out of my way through life not to hurt anybody. My 'H' was a drink fuelled roller coaster with jealous rages (through out our relationship) and screwed me up so badly, (bad metaphor) I am on AD's and having counselling. I also fell (very sadly) for someone else.
I have done nothing about it. I do not think this makes me better than your H SBB, because even now there are times when it gets so intense that if I was in the wrong place at the wrong time, I would struggle to keep hold of my morals.
I am awake now because it is driving me mad. And I am alone.
That he tried a site as you describe suggests to me that he just wanted a release. This does not justify or excuse his actions in any way, but it might be a root to what he was doing.
That he and yourself reached this point is sad. I think I am a good woman mostly, and it is bloody sad that I felt that way, albeit for different reasons.
If you want to get passed it, and FWIW, I think you should try to, then maybe in a tiny way, knowing that he just wanted to satiate the need might help you to move forward.
It really is a horrible feeling, and in some ways I wish I could have just gone somewhere and get rid of the need because now I have something far more to contend with. And that is feelings for a guy I shouldn't have. A broken EA marriage and a long way to go before I will be in a position to look at a future that might hold a good, kind loving partner who takes the ache away.