There's actually evidence that the female orgasm does indeed serve a biological purpose, besides making sex more enjoyable for them (especially those whose significant others have mastered the Zen of cunnilingus).  I remember reading something in Robin Baker's book Sperm Wars { http://www.amazon.com/exec/obidos/tg/detail/-/0465081800/002-5900234-3744845?v=glance } about females having greater retention of sperm after having an orgasm.  So I Googled "female orgasm greater retention of sperm" and found a bit about it in an article entitled "The Orgasm Wars" in Psychology Today....

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Sperm Competition, with Women Judging

Clues for a reasonable adaptation hypothesis were readily available by the late 1960s, when The British Medical Journal published an exchange of letters about the muscular contractions and uterine suction associated with women's orgasm. In one letter, a doctor reported that a patient's uterine and vaginal contractions during sex with a sailor had pulled off his condom. Upon inspection, the condom was found in her cervical canal! The doctor concluded that female orgasms pull sperm closer to the egg as well.

Yet, it was only three years ago that two British biologists, Robin Baker and Mark Bellis, tested the so-called upsuck hypothesis. They were building upon ideas articulated by evolutionary biologist Robert Smith, who suggested that since women don't have orgasms every time out, female orgasm favors some sperm over others. Baker and Bellis sought to learn just how female orgasms might affect which of a lover's sperm is used to fertilize a woman's eggs.

They asked volunteers to keep track of the timing of their orgasms during sex, and, after copulation, to collect male ejaculates from vaginal flowback--a technical term denoting a distinct form of material that emerges from the vagina several hours after sex (scientists have devised a way to collect it). The team counted sperm from over 300 instances of human copulation.

They discovered that when a woman climaxes any time between a minute before to 45 minutes after her lover ejaculates, she retains significantly more sperm than she does after nonorgasmic sex. When her orgasm precedes her male's by more than a minute, or when she does not have an orgasm, little sperm is retained. Just as the doctors' letters suggested decades earlier, the team's results indicated that muscular contractions associated with orgasm pull sperm from the vagina to the cervix, where it's in better position to reach an egg.

Baker and Bellis proposed that by manipulating the occurrence and timing of orgasm--via subconscious processes--women influence the probability of conception. So while a man worries about a woman's satisfaction with him as a lover out of fear she will stray, orgasmic females may be up to something far more clever--deciding which partner will sire her children.

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source: http://cms.psychologytoday.com/articles/pto-19960101-000028.html