Female orgasm eludes evolutionary explanation.  Most candidate explanations have a hard time reconciling the observation that a large fraction of women do not have orgasm during intercourse and among those that do it is not a consistent occurrence.  Here is a fun paper surveying a variety of just-so stories that “explain” female orgasm.  The authors dispense with

  1. Its a non-adaptive vestige of male orgasm.
  2. It encourages females to have more sex. (then why not always?)
  3. It encourages females to have sex with multiple partners (thus the asymmetry in “arrival times” between males and females.)
  4. It improves chances of fertilization. (empirically false)

and they leave us with an intriguing, relatively new one, the Evaluation Hypothesis.

When Barash was a graduate student  more  than  ten  years  earlier,  he  observed  that  when  subordinate  male  grizzly  bears  copulate,  their heads  are  constantly  swiveling  about  on  the  lookout  for  a  dominant  male,  who,  should  he  encounter  a  couple  in  flagrante,  will  likely dislodge  his  lesser  rival  and  take  its  place.  Not  surprisingly,  subordinate  males  ejaculate very quickly, whereas dominants take their time. If female grizzly bears  were to experience orgasm, with which partner would you expect it to be more  likely? And is it surprising that premature ejaculation is a common problem of  young, inexperienced men lacking in status and self-confidence? Moreover, is it  surprising that women paired with such men are unlikely to be orgasmic?

So it doesn’t encourage more sex uniformly, it encourages more sex with the right mate.  And it is inconsistent and slow to arrive, not by accident, as in the vestigal hypothesis, but by design.  And the sorting of men according to, let’s call it patience, seems to be a stable equilibrium as it requires either an exogenous characteristic correlated with “good genes” as in the case of dominant grizzlies, or perhaps in its social incarnation where it requires

sufficient  access  to  resources  to  orchestrate  interactions  that  are  private,  safe,  and gratifying—in a word, romantic—and thus appealing to women’s evolved evaluation  mechanisms.

From the book How Women Got Their Curves and Other Just-So Stories:  Evolutionary Enigmas by David Barash and Judith Lipton. (Cloche Click:  Bookslut.)