People eating what is usually called food should be a minority because there are so many other things that fit in your mouth: stones, grass, computer components ...

For this to be a relevant analogy, we need to have adequate reason to think that monamory, like food, deserves its privileged position out of possibilityspace. There are specific, overwhelmingly powerful reasons to think that stones, grass, and computer component are inadequate sources of human nutrition; but in the absence of such considerations, it would certainly be unreasonable to simply assume that what we've always eaten is the best thing we could possibly eat out of some set of options.

My claim isn't that no possible evidence could ever show that monamory is better than polyamory. My claim is only that in the absence of strong evidence in either direction, we should expect polamory to win, for the same reason we should expect 99 randomly selected stones to have at least one stone that's shinier than another 1 randomly selected stone. Some positive argument for monamory is needed; whereas no positive argument is needed to privilege polyamory, so long as it permits orders of magnitudes more varieties of human behavior than does traditional monogamy. It's because no one specific behavior has yet been shown to have a privileged amount of utility that the broader category gets a head start.