How to bring up divorce

My partner and I have been married for 2 years, together for 4. I caught them getting pictures from those OF/Telegram models-I know they paid for the pictures but still hurts and crossed a major line with me. I’ve lost trust and honestly don’t even think they would stop if I asked them to. They know it hurt me and didn’t see it as doing anything wrong. How we got married and were this misaligned, I have no idea. I’ve been thinking of divorce as I’m not one that can just “get over” this-I consider it cheating. I would love to try couples therapy but they’ve already shot that idea down. I feel like divorce is my only other option. Any recommendations for how to initiate this conversation? It sucks because when things are good, they’re great and I hate that they always seem to be having a great day when I’m ready to bring it up-I don’t want to ruin it for them. I hate that I’m like this sometimes.

tl;dr
How to bring up divorce after continuous inappropriate pictures with OF models