This comment seems... fundamentally confused. It seems like it addresses me directly, so I'll reply instead of ignoring it.

I'm fairly sure I can't be satisfied being anyone's second-best, or even one of three who are rated approximately equal (though the latter doesn't bother me as much as the former).

This seems to be something about you. If that's not something you're comfortable with, go ahead and don't enter into relationships like that.

When you have a "primary" but still include other people who aren't your "primary", you're demanding to be given something---priority---that you yourself won't give. You're asserting that you have a right to demand special status, but other people don't.

My goodness. My primary has a right to special status from me because I have the same from him. If we were monogamous, we'd be "egalitarian"ly so; but then my other boyfriends wouldn't get to date me at all. I think this would upset them. Neither of them are counseling me to dump them so I can commit fairly to my primary. (Or begging me to run away with them instead, for that matter.) But note that I can only "demand" special status from my primary because he's okay with that. I did not break into his house and say "I must take first priority in your love life, or else!" It's a thing we decided to do with each other. He and I can meanwhile maintain secondaries who may seek primaries or flings or whatever-they-want elsewhere too (or not, if they don't care to). This is all just more people having more options.

The only way I can see to soften that blow is to say that some people don't want to be number one; they don't mind being second-best. But first of all, I find that a little hard to believe to begin with (who wouldn't want to be the favorite if they could be?)

Someone with limited time, who doesn't want to be wanted more than they can find opportunity to be available? Someone less extroverted than their partner, who needs large swaths of alone time? Someone with a primary of their own elsewhere? Someone who gets off on not being treated as the favorite? I have yet to sincerely underestimate human heterogeneity.

and second of all, frankly I have trouble imagining that I would be compatible with someone who thinks that way.

This seems to be something about you. If that's not something you're comfortable with, go ahead and don't enter into relationships like that.

For this reason, I can really only see two morally justifiable modes

You've made a leap from your own psychology to ethics in general.