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"