I'm built for monogamy. For me, the prospect of being with my loved one and screwing a bunch of other people is depressing. I was with one partner who told me after we'd been together for a while that he wanted an open relationship so we went our separate ways. The man I met after that wanted what I did.
I was raised on 2 hippie communes surrounded by naked people practicing monogamy, polygamy, open relationships, bisexuality, homosexuality, heterosexuality...it was all right there in front of me and they were all my loved ones. I'm fortunate not to have any religious baggage and I was exposed to many kinds of relationships.
Years ago I had a group of friends who hammered me for my beliefs in monogamy, telling me there was something wrong with the way I lived, heaping healthy dollops of judgment because I was different than them. Where are they now? Every last one of them is dead.
Open relationships aren't "natural" for everyone. One size does NOT fit all. None of this is a moral issue, it's just what feels right. And yes, there are lots of other men who believe as I do; water seeks its own level.