I have had the unwanted privilege of watching the polyamory* movement grow before my very eyes, in friends, and in movements. It has become so strong that it helps define movements. I am an anarcha-feminist, so I was very disappointed to see an article on the anarcha-feminist community about "saying no" to monogamy, but none about being a strong, monogamous female. There is a big difference between being radical and denouncing something because of its inherent flaws, and being pseudo-radical and denouncing something because it is the status quo, and therefore must have been created by patriarchal society.
There are two key parts to this question:
1) Monogamous relationships and
2) Marriage, as defined by Western society.
The first is perfectly natural, and can be found in human beings and other animals, including some mammals. It is not about ownership, or power struggles. It is about finding a reflection of yourself, a soulmate, and for some people there is only one. I have the right to choose to be monogamous, just as I have the right to choose anything else as an individual woman.
The second is created by a patriarchal society, and possibly has an original purpose of tying the woman to the man, so he is the sole owner of her children. But that's how it is defined in Western culture, not how it has to be. My parents' marriage is incredibly egalitarian; my mother did not take on my father's name, and I have both of their names. They share in the work, although my father does cook more, because he enjoys it. My mother has a private practice, my father is a teacher. They are both strong feminists who don't shave :)
I understand that being a minority in this culture, polyamorists feel anger toward the status quo, just as homosexuals may feel anger towards heterosexuals, blacks towards whites, and so on. But that does not mean that monogamy is unnatural, just like heterosexuality isn't unnatural, and whites aren't evil. (This is a simplification; I am very aware of homophobia and racism). Just like striking out toward the oppressor isn't necessarily striking out toward all whites, or all heterosexuals, you don't denounce monogamy as a whole because a few monogamists sneer at polyamory.
Moreover, claims that monogamy has been proven to be destructive and not work, are completely unfounded, considering that I have yet to find a stable polyamorous relationship. This in no way suggests that polyamory is inherently unstable; it merely points out that humans have relationships, some of which do not go smoothly. If you have trouble in relationships, be it power dynamics, insecurities, jealousy, etc., those problems will show up in a monogamous or non-monogamous relationship. Just like it is irrational to seek a lesbian relationship because you have trouble with men (even though you're attracted to them), you cannot seek polyamorous relationships just because you suck at monogamous ones. In my opinion, you must truly feel in your heart that you will be happier seeing more than one person. Some people become polyamorous because of fear of commitment. They say that humans are naturally sexual, loving creatures and therefore need more than one partner. This is bullshit. If Sally says humans are naturally non-monogamous, then it's because Sally is non-monogamous, and is confusing herself with the entire human race.
People like to blame the entire human race for their own natures. Some of the most angry, bloodthirsty people I know say that violence is human nature. Yet it is their nature that they're talking about. Humans are naturally:
That is true human nature.
I will end with four points, two of which I have made before:
Humans are naturally peaceful.
Humans are naturally violent.
Humans are naturally monogamous.
Humans are naturally polyamorous.
The "human nature" argument gets nowhere past me!
*Polyamory is a descriptive term (defined by contrast with monogamy) for the practice of being open to more than one loving, intimate relationship at a time, with full knowledge and consent by everyone involved. (Wikipedia)