I remember hearing what a huge journey it is for the sperm to reach the ovum, as they are so small and the journey is the equivalent of a human traveling hundreds of miles (or whatever). I recently saw this, and it made incredible sense: the female orgasm is a selector. It causes internal contractions that not only draw the sperm up the path at break-neck speeds, but even draw it up the correct path (as there are two ovaries and only one is producing at any given time). I don't think we can say it's merely a vestigial sensation, because I think females are the original gender; the penis is just an adapted clitoris after all.
http://andrewsullivan.thedailybeast.com/…
http://andrewsullivan.thedailybeast.com/…