Interesting question you pose. First let me just say, Everyone deserves happiness. Next, Each couple negotiates their own relationship rules. If you can agree on said rules, and are fulfilled and happy within said relationship, great. Problems arise when one or both parties are dissatisfied with said rules and a new mutual agreement can not be negotiated. So If your relationship allows for extra curricular sex, it's not cheating, provided you stay with in the confines of stated rules. If monogamy is essential to the relationship, and one or both parties breaks the monogamy rule then yes, it's cheating. Two very basic emotions, we as humans, get from any relationship: 1. To feel good about ourselves when with another person, 2. Not to not feel good about ourselves when with another person. One is healthy the other toxic. Which do you want? I choose 1, however that plays out.