What is her definition of stifling?....whats her history in this ? Is she married or have a primary relationship?
Keep in mind that this is my interpretation of what she's told me, or said to others. I'm sure the "game of telephone" applies...
She was once married, about a decade back. She found her husband/marriage stifling in that she felt she lost herself to make him happy, that she did not have the "alone time" she wanted to pursue her interests (non-romantic) - that he basically smothered her.
They divorced, she was single for a number of years, got introduced to the concept of Poly, and not long after, started a relationship with our current partner. We are probably both co-primaries, for lack of a better term. He currently splits his time between two separate homes.
My tack on this: Yes, a Poly relationship lends itself to having more alone time for you if you do not have multiple partners. Yes, a smothering partner is probably not going to want their love to be non-monogamous. Therefore, anyone willing to get into a Polyamorous relationship may be a bit less "stifling" to begin with.
So, despite her belief that marriage and monogamy is stifling, I don't believe it's the fault of monogamy or marriage itself as an institution, but can be blamed on personal incompatibilities with the person you choose as a partner. A controlling partner can certainly (mis)use marriage as a tool in order to BE controlling. She had a husband who wouldn't give her any time to herself. A different husband may have given her the freedom she desired to pursue her other interests within the context of a monogamous marriage.
Anyhoo... I find the difference in perception (and possibly semantics) interesting, although I do get tired of the "monogamy is <insert negative word here>" that I hear from time to time.