Potential long post incoming, so apologies in advance.
I personally believe that polyamory and monogamy are completely unrelated things. I think polyamory is the personal capability of someone to love more than one person, and I think this actually applies to all humans - that is to say that every human is capable of loving more than one person at a time. Just like the oft-used example of loving more than one child without loving another any less, I think that a person is able to love more than one person, but through a combination of social pressure and learning in the vast majority of cases, and where people who have come into contact with polyamory and have questioned their own beliefs about relationships etc and still prefer monogamy, I think that is a case of still being able to feel love for more than one person at once, but their DESIRE and PREFERENCE is monogamous arrangements.
There is huge social pressure against cheating, promoting monogamous arrangements and yet we hear of infidelity constantly. Now, yes, in some cases it's purely sexual, but we also hear about "emotional infidelity" where there is no sex involved, or the sex is incidental. To me the answer to this is because humans are naturally polyamorous, and to the "uninitiated" (i.e. anyone who hasn't examined their own love life to work out what it is that they want from it OR people who enter monogamous relationships as a form of control on their partner to protect their own feelings) there is conflicting desire between their promised monogamous relationship and their natural ability to love more than one person at once. I'll talk about people who HAVE examined their own lives as regards monogamy soon, once I define what I think monogamy is.
I believe that monogamy is a relationship SYSTEM - i.e. (generally) exclusive, one-one matches etc. We call it polyamory when we follow any of the huge number of possibilities that exists outside of monogamy, but I think non-monogamy is a better term, since as I explained above, I believe polyamory relates to our ability to love more than one person, rather than how we structure our loving relationships. If you're having trouble understanding this point, then try and define a polyamorous relationship... you can't do so without saying something like "it's not monogamy" - there are too many different ways of structuring relationships for it to apply to any one type of things.
I said that I'd mention people who have examined their own lives and still choose monogamous arrangements, or even define themselves as monogamous people. A frequent message on this site is that love is an infinite resource, but things like time and energy are not. While someone may be ABLE to love more than one person at once, they may only WANT to feel that with one person at a time. Obviously we're not including anyone who only gets into monogamous relationships because that's what they've been taught (although we shouldn't entirely discount the social learning that happens for years, reinforced all around our society - movies, books, tv shows etc) but people who have realised and understand that they WANT and CHOOSE monogamous relationships for themselves. I guess the best way to explain this is to say that while I think that all humans are ABLE to love more than one person, they are not REQUIRED to love more than one person, or even anyone at all.
I believe the difference between monogamy and non-monogamy as relationship systems can be compared to the people who naturally prefer to have a small group of close friends to a larger group of less close friends. While someone who prefers a small group of close friends would certainly be ABLE to add a new friend into the mix, they might not WANT to. Similarly for monogamous arrangements, someone might be ABLE to love a second person, but they don't WANT to. They understand themselves well enough to know that they prefer to invest their limited time and energy into one relationship with one person.
At this point, I'd like to bring up poly-fidelitous arrangements, a sort of half-way house between monogamy and complete non-monogamy - a group of people who only have sex and romantic connection to other people within the group. To me, this actually reinforces the idea that monogamy (as a system) is a chosen system - while it's clear that these people can love more than one person (since the group is more than two) at the same time they're promising EXCLUSIVITY to each other because they, like those who choose monogamous relationships, prefer to invest their limited time and energy into those people within the group.
For all other non-monogamous arrangements, the people prefer to invest their time and energy with multiple people - I don't have to say a whole lot on this, since anyone on this site either knows, or is learning about it
These "belief systems" (I use quotation marks because for some people it's belief, some people know it instinctively and some people actively and consciously seek it out) are subject to the fact that we are all human and therefore make mistakes and don't always know what's best for ourselves. The changes from mono->poly or poly->mono are often brought about by shifts in our beliefs, and a greater understand of who we are. However, it's rare that you hear about people who go through the same process of introspection and/or learning but instead of changing their mind, it REINFORCES their belief in their own system. This is important to note, because I believe that each system is a totally valid way of living your life and managing your relationships, as long as you know that it's right for YOU. Any type of relationship borne out of fear and insecurity is a recipe for disaster.