As a first comment here, I'd point out that defining women independently from solely their role as mothers
is judging them on character rather than identity, so you should support such a thing if that's your rallying cry.
But also, on MLK and postmodernism: postmodernism dates back to the 1940s, To Kill a Mockingbird is postmodern. MLK's Letter From a Birmingham Jail is very clearly postmodern (e.g. when he says "But I am sorry that your statement did not express
a similar concern for the conditions that brought the demonstrations into being.", he's alluding to a failure to consider the viewpoint of the oppressed in the situation).
That letter is also very modern-leftist. Kimberle Crenshaw coined the term "intersectionality" in 1989 and elaborated on it in 1991, saying "When feminism does not explicitly oppose racism, and when anti-racism does not incorporate oppposition to patriarchy race and gender politics often end up being antagonistic to each other, and both interests lose". The letter from a Birmingham jail includes another famous line from MLK: " Injustice anywhere is a threat to justice everywhere. We are caught in an inescapable network of mutuality, tied in a single garment of destiny. Whatever affects one directly affects all indirectly". To me, those express similar sentiments: that oppression exists in many forms in many places, and it is unwise to pretend that oppression that fails to inconvenience me is therefore unworthy of my attention.
And of course he says later on "there is a type of constructive nonviolent tension that is necessary for growth. Just as Socrates felt that it was necessary to create a tension in the mind so that individuals could rise from the bondage of myths and half-truths to the unfettered realm of creative analysis and objective appraisal, we must see the need of having nonviolent gadflies to create the kind of tension in society that will help men to rise from the dark depths of prejudice and racism
to the majestic heights of understanding and brotherhood. " which to me describes the people often criticized as "modern far leftists", those people who create the tension and discomfort in society are, in MLK's view, doing us, collectively, a great service.
I'd recommend you read the whole letter[0]. If you're so willing to lionize MLK, but disagree with so much of what he preached, I implore you to consider why exactly that is.
[0]: https://www.csuchico.edu/iege/_assets/documents/susi-letter-...