Evolution gives rise to complex adaptations, of which behaviors are a sub-set. It may well be that female orgasmic potential did indeed begin as a by-product of the obvious requirement for male orgasm (however, one can ejaculate without experiencing the pleasure of an orgasm, so even this point is debatable...). But it could then have become highly adaptive. Human males and females must pursue different mating strategies because the economics of reproduction are vastly different for males & females. Men can "scatter their seed" widely at almost zero cost, whereas a woman can only bear one (or, sometimes, two) children at a time and then must spend at least four years rearing them until they are viable from a survival perspective. So men can exhibit low choice thresholds (basically anything of child-bearing age with a pulse) while women must be very choosy so as to maximize their reproductive potential.

Unfortunately for women, men with status, power, and wealth, may not have the best genes. Therefore a good strategy is to partner with such a man and then have illicit secret copulations with one or more males whose appearance suggests superior genes. This yields the best of all worlds: great gene mingling along with the resources necessary to raise healthy children.

Research shows that female orgasms are generally stronger during illicite mating than when having sex with one's "official" partner. Thus what began as a by-product could have been retained and developed as a means to enhance female reproductive potential.