If, on the other hand, someone's reason for being monogamous is "because the Bible says so," or because "everyone else is doing it," or because "polyamory is unhealthy," then those are bad reasons for being monogamous.
As with most stances people take in life, which relationship style people adopt has very little to do with conscious decision making. People in the US, for example, are mostly monogamous. It isn't speculation that there is a bias in favor of monogamy because it is the only form of partnering allowed by the state - there is clear and undeniable bias. For this reason we shouldn't be surprised that most people claim to be monogamous and they don't have any idea why.