I completely agree with you, though not that my definition of monogamy is misguided. I do, however, agree that most people in monogamous relationships have not decided that it's what they want or bothered to figure out exactly how a monogamous relationship would play out.
I think that polyamory has advanced the situation in demonstrating what a relationship is, but unfortunately much of it is still handled childishly by those who advocate it as the evolution of relationships.
Relationships in our culture are, for the most part, still handled like children. We look to the adults around us to figure out what to do and then copy their mannerisms. We become confused when the copying of the mannerisms does not result in the happiness demonstrated in fairy tales, advertisements, and select couples.
One of the major problems in my view is that relationships are still considered primarily from the view of emotions. Polyamory is often referred to as the ability to love more than one person, an ability which I think is not an ability at all but simply part of the human condition. Polyamory instead, I think, ought to be framed as the desire to have relationships with more than one person thus creating an entirely different understanding.
Of course, as you accurately noted in your example with Runic Wolf, most of monogamy is also still based on fairy tale ideals of love preyed on by our culture. This creates problems even for those who would choose monogamy. Couples end up in relationships and wonder why love doesn't do it all for them. They also become concerned if there are feelings and desires for others which I think it's impossible that there wouldn't be.
Monogamy thereby is the choice of two people to share a large amount of their resources with one another. This demands a great deal of effort, though hopefully effort that is mostly enjoyed. The point of a monogamous relationship is arguably the opportunity to fully explore another person which means active effort must be put in to do so.
A monogamous couple would have to actively endeavor to share new experiences, go new places, have new conversations, etc. The sheer volume of self-help books on reinvigorating relationships I believe is because of the idea that love carries a relationship. Couples put effort into dating and then sit back and wait for the rest of their relationship to happen. Of course this would result in the stagnation of anything, but the reliance on emotions in understanding relationships clouds the issue.
Additionally, implicit in monogamy is the understanding that you both will have feelings and desires for other people but are choosing to commit whatever amount of resources you choose to each other. Neither romantic feelings nor sexual desires for other people are in any way threats to monogamy.
Ideally I think young teenagers would be taught the entire range of relationship styles in completely practical terms. This would include the realities of the various kinds of relationships, how the day to day life lays out, what's demanded in each of them, and the realities of romantic and sexual emotional responses as well. Unfortunately we're still struggling with even allowing the teaching of sexuality so this would seem a long way off.