Based on your subject alone I was prepared to say something like "You can break up for whatever reason you want, but as someone who's done both monogamy and nonmonogamy, I can honestly say that it doesn't mean he yearns to cheat" ..... but with the context you gave, I'd feel insecure too. Nonmonogamy can be a really cool, fun lifestyle choice to experiment with and some people love it long-term... but he doesn't want "nonmonogamy as a lifestyle choice or a lifestyle experiment," what he wants is to experience single life. In other words he's not thinking "hey maybe there's other ways to have a serious loving relationship besides the traditional ways!" ... he's thinking "I feel stifled and unsatisfied!" I'd be worried in your case, too. Regardless, you can break up for any reason you want; but if you're looking for some validation for how you feel then yes, even I (someone who has tried and enjoyed nonmonogamy!) would feel uncomfortable continuing to date someone who had basically admitted they feel unsatisfied with me / our relationship.