Do you think that the fundamental basis of feminism is sexual choice so as to not be monogamously stuck to a lesser man?
The premise of your question is flawed.
The fundamental basis of feminism is the search for a feminine identity, however and whatever that means. The sexual choice is there NOT so that girls can leave a monogamous relationship whenever they want, the point is vetting like how men do, because women do expect a lot from men, and rightfully so due to the biological imperative.
The obstacle is ego on both sides.
Some girls think they know better on how to properly choose a partner they could choose to be in a monogamy with, but they are nonetheless ignoring childhood trauma, biases, and accountability with respect to the thoughts they have.
Meanwhile, some guys think they can get away with simply carrying themselves and using their frame in appearance, instead of actual improvement and betterment that requires consistent change in one's own ways of thinking. Moreover, a feminine identity is better defined when men themselves know how to define their own identity, such that the maxim "men lead, women follow" is upheld. If men are lost with regards to who they are, imagine how much more lost women are.
This obstacle is persistent because humans, by nature, are afraid of change.
Some of this fear stems from simply not knowing how to handle that fear, mainly because their environment did not allow them the freedom to experience and learn from that source of wisdom.