My advice is to come at this from a non-fandom angle.

Show him that all the “porn rots your brain” etc sex negative “research” actually comes from hyper conservative christian sources.

Make it a human sexuality issue, instead of a fandom issue and maybe he will understand where you’re coming from.

Is this person someone who knows you well IRL, or someone you’ve been dating exclusively on the internet? My advice for how to handle a potential breakup will be different depending on which.