In polyamory, people tend to have a maximum limit of partners, or a point at which they'll say, "Whew! I can't spread my time and energy any thinner." We all agree that love is an unlimited resource, but we admit that time and energy have their limits.
Some may be willing to have as many as ten partners. Some, only five. Some, three, or two. It's not that they *couldn't* spread their resources thinner, it's that they don't *want* to. They want a certain quality of time and effort devoted to just a few relationships.
If someone says, "I want to devote my total amount of available time and energy to just one partner," then we could say that person is monogamous by definition. If their reason for being monogamous is that it's simply their preference not to divide their time and energy between more than one partner, then I find that acceptable as a reason for them to be monogamous. They're no different than a polyamorist who wouldn't want more than two or three partners, their preferred limit just happens to be a smaller number.
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. In theory, we should expect polyamory to describe the larger amount of the general population, as polyamory covers many "maximum numbers of partners," whereas monogamy only covers one maximum number. In practice, we observe that most people become and remain monogamous. This gives us a hint that there's something out-of-balance with tradition and popular culture.
So, for many monogamists, polyamory would be a better choice for them (a choice they probably don't even think about, or aren't aware of). But some monogamists would probably choose to remain monogamous even if tradition and popular culture were corrected -- simply because it's their personal preference (and represents the most partners they'd want to divide their time and energy between). Thus I don't have anything against monogamy in theory, but I'm also aware of a bias toward monogamy in mainstream society, and I take issue with that bias.
Just food for thought ...
Love means never having to say, "Put down that meat cleaver!"