I think there are a few important distinctions about participating in sexual fantasies independently while in a relationship.

For starters is this a kink or a fetish. If it’s a fetish and he absolutely needs it to be fulfilled sexual the chances of it escalating is high and puts you at risk even indirectly (ex. sharing your details or images with others online for role play)

If it is a kink there are healthy ways to explore kinks your partner doesn’t share solo. For example my partner and I both enjoy bdsm but not all the same types of play. One of their hard limits and one of my hard limits are some of the others favorites. We both know this and acknowledge this. We are both completely fine with each other exploring those kinks in private in terms of consuming porn, stories, etc about it. What is not ok is involving either of us or an actual third party (ex online role play) in that kink exploration. I’m never going to be mad at them watching porn about something that’s not my cup of tea. I also cannot police their mind and if they think about me in terms of that kink and that’s ok. I would be mad if they were participating with other people directly online or in person though.

It’s important to note just because a fantasy isn’t shared doesn’t mean the part we has to go outside the marriage to get fulfillment from it. The imagination and existing media can be a very useful tool.

If it’s #1 which it seems like it is, that is very concerning and worth working with a counselor if you wish to stay together or leaving.

If it’s not #1 and it’s a kink that just got out of hand (which can happen when you discover something new and exciting and act selfishly) ask yourself what boundaries you would need to discuss and establish to start feeling secure again.