Suffice it to say mainstream society does have a bias in favor of monogamy, which can exert pressure on some of us to be monogamous. "Well, I don't like monogamy ... so there must be something wrong with me ... so I'll just try to learn to be monogamous and accept it." If a person is influenced by mainstream society in that way, then they have compromised a bit of their autonomy.
At the same time, it's a fallacy (I think) for a polyamorist to think, "I belong to a superior society (the 'polyamorous community'), and anyone who practices monogamy is a slave and/or a slave-owner." Some monogamists may have been pressured into monogamy which puts a ding in their autonomy, but other monogamists have chosen to be monogamous quite on their own free will, and thus their autonomy remains intact.
I guess the point is that mainstream society's bias affects the percentage of monogamists who have a ding in their autonomy, however it is not monogamy's fault per se that this ding occurs; rather it is the "fault" of the bias that mainstream society has. (That combined with the tendency some of us have to let mainstream society push us into things.)
Many polyamorists have also developed an "us versus them" mentality, and have reacted to the imbalance mainstream society contributed to by formulating their own "counter-imbalance" (namely, the idea that all monogamy is bad). "They say all polyamory is bad? Well, we say all monogamy is bad, so there!"
The solution, then, in my mind, is to try to practice more tolerance overall between the monogamous and polyamorous "groups." Just as we hope being "gay" or "straight" can be considered equally valid options, so it would be nice if everyone could think of "monogamous" and "polyamorous" as equally good options (assuming all else be equal).
Hope that wasn't too incoherent.