As long as we are using monogamy to mean: romantic exclusivity (sexual and emotional), I disagree that monogamy does not preclude autonomy.
I did not say that monogamy does not preclude autonomy. I said it does not
necessarily preclude autonomy. Losing or giving up one's autonomy is not an across-the-board requirement of all monogamous relationships. How could it be? There are many flavors of monogamy, because individuals are involved - why do poly people think they are the only ones with variety?
Any relationship configuration is going to depend on the people involved. Yes, many people do make demands on their partner to be exclusive; and yet for others, both partners willingly and happily embrace, invite, and accept exclusivity. Still, exclusivity does not negate a person's autonomy. An individual can be autonomous, be their own person, have the freedom to express their individuality, live life in their own way AND YET happily choose to be monogamous. There is no rule that these things have to be abandoned to be monogamous.
Personally, I've had many monogamous relationships where I did not give up my autonomy, and I have observed the same in many other monogamous couples where they were loving, mutually supportive, and free to express who they were. It's a shame that so many poly people don't seem to have observed much of that, or do they just refuse to see it? ( <-- more likely) Monogamy and polyamory are just relationship structures, neither of which is the be-all-and-end-all key to happiness. Happiness and autonomy can be had in nearly any relationship configuration - it's the person who makes the difference, not how many people they love or have sex with that determines how much personal power they retain, own, express, etc.
Sure, a monogamous relationship can be fucked up and the people in it utterly co-dependent, but that can happen in polyamory, too. Polyamory does not
necessarily encourage autonomy just as monogamy does not
necessarily prevent it. Your relationships are what
you make of them, not what the structure and dogma of a particular culture make of them.