100% not judging here, but it surprises me how often I hear about long-term gay male couples that start out monogamous and then fall into non-monogamy, as opposed to just starting out non-monogamous. Perhaps a large portion of gay male relationships start non-monogamous but don’t last; I don’t have know of any data on that.
One way to look at it is that a monogamous foundation is needed for a long relationship. Another way is that we’re indoctrinated in monogamy and sleeping around saves relationships. All I know is what’s true for me, which is that an open relationship didn’t feel necessary at first, then it did.