You don't want to lose your relationship, and you don't want to feel stuck in this place, either. There's nothing wrong with feeling attracted, even strongly attracted, to another person.
Some couples have fun sharing their stories of attraction and desire.
Others just want to be open to all that they are feeling without acting on it, to see what they can learn about themselves. Sometimes, it's just fun to indulge in fantasies without worrying about what they mean. It's possible for a relationship to last after an affair, but whether it really heals or not is another question. It's very tempting to try to put an affair behind you, but the reality is that it will continue to surface in different ways in your relationship if it's not adequately addressed. When couples take the time to talk it through, usually many times, and offer true understanding to each other, they have a better chance of moving on in a real way.