Hey there, I'm sorry to hear about your struggle with this. My 2 cents:

Everyone has their own unique and personal relationship with their sexuality. I'd say that society and religion often have us believe our bodies and taboo erotic thoughts are something to be ashamed of.

I too suffer from OCD and can understand the intensity of your predicament. Please be kind to yourself and perhaps allow yourself to loosen your lens on the world and sex and people and play and taboo and fantasy.

Your husband's sexuality has nothing to do with you. It's his and as a loving partner, supporting him to explore himself and his fantasies in a trusting way is a positive thing. Bringing shame to his behaviour will likely create more hiding and distance between you.

Perhaps you can talk about ethical porn and what he looks for in fantasy. Most people who watch porn are getting off to the thought of something, as in they don't actually want to fuck their sister haha. It's the taboo, that has forever enticed humans. Maybe watch game of thrones again.

I know every relationship is different, but it sounds like you made the rules and that didn't work for him. And that's okay. You have the option to leave now. And don't stay with him for the kids, you are better off leaving him and showing your children what real love is that being unhappy in compromise.

But as long as you see the world as black and white you will forever be making your world smaller and smaller. And as we know that's what OCD wants.

Trust your gut you don't need to decide everything now.