Andrew Sullivan linked to a study seeking the evolutionary basis for the female orgasm.
.....female orgasm may thus be a relic, adaptive among our primate ancestors but potentially disadvantageous—even dangerous—to some women today. Thus, insofar as orgasm might even occasionally induce women to seek out additional sex partners beyond their designated husband, this consequence in itself might have serious (and certainly fitness-reducing) results. In much of the world, the penalty for a woman’s having sex with more than one man (especially if she is married) is quite severe, sometimes including death.
Of course as a man I'm wildly speculating, but in the past (and often in the present) the risks of pregnancy and childbirth and the stress and responsibility of raising children made sex a much higher-stakes proposition for women (hence the revolutionary societal impact of the birth-control pill). It seems to me that from a purely biological perspective the female orgasm has been perhaps even more important as small - indeed woefully inadequate - compensation for what historically has been an act fraught with peril and huge potential long-term downside. A man didn't (and still doesn't) need much reward for sex, but for a woman that orgasm better have been good to be worth the risk.