I'm pretty sure almost everyone has sexual thoughts and fantasies about other people when they're in a relationship. There is nothing wrong or scary about this, I've always been like that and have never been even close to cheating on any of my partners. Cheating is a choice. Even if you're so drunk you almost can't get it up, it's still a choice. It's not fair to expect the person you're in a relationship with to only think sexually about you. But it is definitely fair to expect him to be faithful. If he feels being faithful is difficult, it's probably not because he fantasizes about other people, but because he has some other issues either with himself or with the current relationship. So I wouldn't worry too much about this, but if you are wondering if he really is the kind of person who's able to stay faithful in a life-long relationship, it's probably a good idea to talk about this so you can know what's going on and maybe be proactive in some way if that helps you.