We actually talked about this in group therapy once. If they want to cheat, they will. Worrying an being jealous won’t help it. I remember an episode in dr Phil, where a guy was sure his girlfriend/wife was cheating on him, so he tracked her and made unexpected drop ins at home. He never caught her. They asked him what he was going to do if he ever caught her. Nothing, he wouldn’t even leave her. He just wanted to know. She obviously wasn’t cheating he was just so insecure but more often than that, the paranoia won’t even result in anything, because even if it happens that they were right, they aren’t going to do anything. If you’re so jealous and insecure that it affects your relationship maybe you should try working on yourself instead of controlling the other.