No points for "because they can." David P. Barash, Ph.D. and Judith Eve Lipton, M.D. search for the evolutionary basis of the female orgasm (because they sure won't find it in Aquinas):
For a distinctly nonsexual example of a trait’s adaptive significance changing over time, consider feathers. It is clear from the structure of fossil dinosaurs and birds that the earliest feathers did not evolve in the service of flight because the earliest feathered reptiles lacked hollow bones and were for the most part earthbound. Far more likely, feathers helped primitive “birds” to keep warm and only subsequently became elaborated as aeronautic devices. Among modern birdsespecially malesfeathers are further adapted as sexually selected traits (remember the peacock). A trait’s current adaptive significance can therefore be quite different from its original function.
Even if female orgasm evolved as a mechanism that induced women to mate with multiple mena questionable if intriguing hypothesisit doesn’t mean that human cultural traditions would necessarily welcome it. Thus, the hideous practice of “female circumcision,” still widespread in much of northern and eastern Africa, may owe its existence to a recognition that female sexual desire can lead to multiple partners: in order for a woman to be considered marriageable, it is necessary to guarantee her fidelity by curtailing her orgasmic potential, if not eliminating it altogether.
Hrdy has made the interesting proposal that female orgasm may thus be a relic, adaptive among our primate ancestors but potentially disadvantageouseven dangerousto 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.