Why would a human female have a sexual display despite male biased operational sex ratio?
Sexual arms race.  

There is a fine and internally consistent theory that this strategy by females evolved to oppose the strategy of infanticide by males.  Males of many species including primates and possibly humans can increase their own genetic fitness by killing infants that are not their own.  This frees up attention and physical resources of the female to instead have the baby of the infanticidal male.  Lions are famous for this.  But this is in opposition to the genetic fitness interest of the female, who just invested a bunch of efforts gestating and rearing an infant who was then killed.

My favorite writer on this is Sarah Hrdy.  From the Wikipedia article on this\


  This is where the idea of sexual counter-strategies comes into play.
  Hrdy theorized that by mating with as many males as possible,
  particularly males who are not part of the colony, mothers are able to
  successfully protect their young, as males were unlikely to kill an
  infant if there was the slightest chance that it might be their own.


This explains why women have the sexual displays referred to in the OP.  Other aspects of female human biology match.  Humans keep big breasts so they always look like they are lactating (compared to other animals) - to keep males guessing.  There is no external sign to indicate when a human is ovulating.  Females of other species are receptive and mate when they are ovulating - humans can mate any time.

A strategy of keep the guys guessing.  If one time he had sex with a given female, probably he shouldn't kill the kid.  Maybe it is his?

An interesting (&amp; extremely arguable!) corollary to this is the premise that human culture in its entirety is the male countermeasure to this prehistoric female defense against infanticide: that male dominated culture is a method to ensure paternity.