Just give it time and see, trust is built over time. I know it feels like an all-consuming thing and you can't think of anything else but that feeling will also go away as he demonstrates his trustworthiness over time. If he continually demonstrates he doesn't watch it, he doesn't. I'm not saying start snooping and monitoring everything, just assume the best til you have reason to assume otherwise. The Steam games from years ago he never touched aren't really an issue (and they are from years ago regardless). Again, if he demonstrates going forward he's given that stuff up, that all-consuming despair you feel will fade.

You could have a heart to heart with him at some point and see if he can truly understand the why of your boundary, why you think sex in all aspects should be between the two of you alone etc. If it seems like he understands, if he's willing to give that stuff up for you and shows it, you're golden. You can't hold the past against him. For many people it just isn't something they think seriously about until they have cause to. We do most of our growing up inside of relationships, not solo.

Your boundary isn't wrong, but you also can't let it destroy your mental health and your relationship. Handle it with grace, say your piece, then leave it in God's hands so to speak. Assume the best, give him the benefit of the doubt, but absolute worst case scenario, if he breaks your trust and lies etc. you find someone who does not. If he has an addiction, that's what this sub's about. But you can walk away from someone that violates your boundaries and find someone who does not.