If you want monogamy then you should get married. Technically you can’t be “cheating” unless you have clearly and legally and unmistakably agreed to be exclusive. So, if you’re not married and have been cheated on, I would beg to differ. But basically, single people should not act as if they were married going out on these ultra long term relationships for 3–5 years on average until an ultimatum is leveled. Set some limit on how long you see someone before ultimately deciding what the long term will be (maybe six months). Marry the person or keep looking or just have fun. There’s nothing wrong with going out but keeping your options open. If somebody that you’re potentially interested in takes an interest, don’t use the old “I got a boyfriend/girlfriend” excuse. You are still allowed to explore and evaluate the best partner for you.