I think that this is cheating. Any behaviour at all, online or face to face, involving other women, that he has deliberately kept secret from you, is cheating. Just because there is no physical contact is neither here nor there.
Are his fantasies taboo because they are illegal or involve abuse or non consensual elements? That would be a no-brainer for me to end the relationship.
‘Taboo’ is a strong word, but depending on your definition might for some people include consensual sexual fantasies in which you would not want to participate personally, but others think of as a kink of some kind and not that far from ‘vanilla’.
However, if everything else is good and you have a long happy marriage otherwise, there is no shame in taking some time to consider whether for you (not randoms here telling you what to do) this is something you want to try to heal from and give him one more chance, or not.
He needs to take full responsibility for this, he chose it, you didn’t drive him to it. If he has a sexual fantasy he can’t do without and you don’t want to participate, then he should have told you that before he did this, and allow you to choose whether or not you wanted to continue the relationship and allow him to pursue the fantasy without you, or for you to choose ending the relationship with him. None of this is your fault.
He needs counselling, if he is a sex addict he needs help with that. Porn/ sex addictions are hard to quit, but I do think it’s nonsense that it must mean they don’t love their partner. Cheating hits happy couples and unhappy couples, it’s more about the personal issues and need for validation and attention of the cheating partner, than perceived relationship or partner deficits. Those are usually cited and exaggerated as their pathetic excuses to any affair partner (and to themselves) for their shoddy selfish behaviour. Very few who have affairs or cheat ever want to actually leave their primary relationship.
If he is asking you to join him in counselling to ‘help him understand’ then you need to ask him what he means by that. He needs individual counselling to help him understand his issues and to understand why he does this, and to help him stop doing it in order to be a safe partner for you.
You don’t need to help him understand, you need him to promise to get help and never, ever do this again. You need him to promise to do anything you need him to to help you feel safe and trust him again. He’s rendered his word worthless and has an absolute ton of work to do and transparency to give you before you can even contemplate reconciling.
I do think he’s appalled at himself and ashamed, as well as sorry for himself. As long as it was a secret he could kid himself it wasn’t so bad and what you didn’t know wouldn’t hurt you. The bubble burst and reality shone it’s light in and forced him to see all the things he hates about himself and how they actually impact you. It’s a horror show.
Even if he does all of the above, you still might not be able to get past this and wish to end the marriage that’s perfectly understandable too.
The point is that it’s up to you and nobody else, don’t be internet-shamed into upending your life, it’s your life, you have to live with your decisions for good or bad, nobody else’s.
In the meantime look up somebody called Michelle Mays, she is brilliant on the psychology of betrayal and sex addiction and there’s useful stuff there. Also Surviving Infidelity and Affair Recovery websites are very helpful. They don’t push for reconciliation at all, they tell you what needs to happen next and provide support and information for those who want to try to get past it, and also support for those who want to end their relationships. They would not advise giving second chances to those who do not meet the criteria for trying to be a safe partner.
Take great care of yourself, this is a horrible shock, I am so sorry this has happened to you.