If he’s been romantic / sexual with them before, then that’s not a genie you can put back in the bottle. You either trust him to not act on the feelings or you break up with him. You can’t control how people feel.
But if you work on a deeper relationship and connection then all thoughts of cheating start looking silly. You do this by trying to understand someone else, the parts they’re conscious of and the parts they’re not conscious of, and accepting them for who they are completely.
If you can do this right, they’ll open up to you because they’ll feel really close to you. You’ll become privy to all their deepest secrets. Eventually you’ll be able to read them like a book. Every little glance, every word, you’ll get crystal clear insight about.
If you can then open up to them, allowing them to see the weirdest and silliest parts of you, then they will know you the same way.
You won’t be able to hide anything from each other anymore, this is very weird and upsetting for a lot of people. If you can get through this and stay together, the idea that either of you would really throw away your relationship is just the silliest idea in the world.