So you just ignore history?
And your mind is certainly failing you if you think "only a few" used them. Deadwood, High Noon, Lonesome Dove, The Magnificent Seven, True Grit, The Searchers, Unforgiven all had prostitutes as major or minor characters.
I'm not saying "two wrongs make a right," but it's just completely fallacious to claim "only a few" Westerns feature them.
who is the prositute in the searchers? even as background character? generally the feminist discussion around the searchers is about natlie wood being stripped of her decsion making abaility and which is different trope.
RDR for better or worse doesn't really look at transgressive westerns like forty guns, johnny guitar and ignores the revivalist (post-searchers, post-western, acid) westerns of new hollywood.