I have agreed to be monogamous in two cases where I would rather have stayed polyamorous, because these girls wouldn't accept it. It was a take-it-or-leave-it situation, and I 'took it' in these cases.
This is a generalization, but men who can stick to their principles are generally more attractive.
Look at it this way: if you can actually "get away with" having relationships that meet your preference, then this is social proof that you are being judged valuable enough ("in the marketplace") to be worth having non-exclusively.
Conversely, if you accede to a request for monogamy, this is evidence that you do not consider yourself that valuable, or that you are unable to get other people to agree with your value assessment.
In short: acceding to a request for monogamy in overt contradiction of your preference is a statement of low self-esteem/confidence, and would be expected to reduce your attractiveness even to the person who made the request for monogamy.
Did the passion in those relationships increase or decrease following your concession? I would guess it decreased, and by more than would have occurred had you not made explicit your preference for polyamory.
If you want polyamory, you'll have to make it a principle, not a preference, and (IMO) state it before someone is even in the position of considering a relationship with you. In this way, merely interacting with you expresses a tacit commitment to at least consider it, and as your perceived attractiveness increases, so will the apparent reasonableness of your principle.
And, your attractiveness increases with your perceived willingness to sacrifice for your principles: this is a highly-valued trait, and a big part of why firefighters, soldiers, doctors, etc. are considered more attractive (than the same person without the role), even though an aspect of their sacrifice is decreased availability to their mates.
Edit: fixed typo of "over" for "overt"
Something like this happens in Dorothy Sayer's novel "Strong Poison", and I gather it happened in the author's life (but she couldn't talk about it, it being the 1930s). Man demands non-marriage relationship, woman gives in, man later concedes to marriage, woman flips out and leaves him.
Many of us are familiar with Donald Rumsfeld's famous (and surprisingly useful) taxonomy of knowledge:
But this taxonomy (as originally described) omits an important fourth category: unknown knowns, the things we don't know that we know. This category encompasses the knowledge of many of our own personal beliefs, what I call unquestioned defaults. For example, most modern Americans possess the unquestioned default belief that they have some moral responsibility for their own freely-chosen actions. In the twelfth century, most Europeans possessed the unquestioned default belief that the Christian god existed. And so on. These unknown knowns are largely the products of a particular culture; they require homogeneity of belief to remain unknown.
By definition, we are each completely ignorant of our own unknown knowns. So even when our culture gives us a fairly accurate map of the territory, we'll never notice the Mercator projection's effect. Unless it's pointed out to us or we find contradictory evidence, that is. A single observation can be all it takes, if you're paying attention and asking questions. The answers might not change your mind, but you'll still come out of the process with more knowledge than you went in with.
When I was eighteen I went on a date with a girl I'll call Emma, who conscientiously informed me that she already had two boyfriends: she was, she said, polyamorous. I had previously had some vague awareness that there had been a free love movement in the sixties that encouraged "alternative lifestyles", but that awareness was not a sufficient motivation for me to challenge my default belief that romantic relationships could only be conducted one at a time. Acknowledging default settings is not easy.
The chance to date a pretty girl, though, can be sufficient motivation for a great many things (as is also the case with pretty boys). It was certainly a good enough reason to ask myself, "Self, what's so great about this monogamy thing?"
I couldn't come up with any particularly compelling answers, so I called Emma up and we planned a second date.
Since that fateful day, I've been involved in both polyamorous and monogamous relationships, and I've become quite confident that I am happier, more fulfilled, and a better romantic partner when I am polyamorous. This holds even when I'm dating only one person; polyamorous relationships have a kind of freedom to them that is impossible to obtain any other way, as well as a set of similarly unique responsibilities.
In this discussion I am targeting monogamy because its discovery has had an effect on my life that is orders of magnitude greater than that of any other previously-unknown known. Others I've spoken with have had similar experiences. If you haven't had it before, you now have the same opportunity that I lucked into several years ago, if you choose to exploit it.
This, then, is your exercise: spend five minutes thinking about why your choice of monogamy is preferable to all of the other inhabitants of relationship-style-space, for you. Other options that have been explored and documented include:
These types of polyamory cover many of the available options, but there are others; some are as yet unknown. Some relationship styles are better than others, subject to your ethics, history, and personality. I suspect that monogamy is genuinely the best option for many people, perhaps even most. But it's impossible for you to know that until you know that you have a choice.
If you have a particularly compelling argument for or against a particular relationship style, please share it. But if romantic jealousy is your deciding factor in favor of monogamy, you may want to hold off on forming a belief that will be hard to change; my next post will be about techniques for managing and reducing romantic jealousy.