One interesting hypothesis as to why the female orgasm is necessary in the evolutionary sense:
It turns out that during the female orgasm, intravaginal pressure has a tendency to rise, but intrauterine pressure has a tendency to decline sharply. With the pressure lower in the uterus than in the vagina conditions are more favorable for the sperms' journey towards the ovum. Thus, it is possible that the female orgasm actively accelerates this journey.
This relates to the two primary orgasmic centers of a woman's body, the clitoris and the vaginal wall. (Women may also experience a "blended" orgasm involving stimulation of both centers.) During the vaginal orgasm, the intrauterine pressure experiences a more pronounced drop than that which occurs during clitoral orgasm. Interesting then, that women who have had vaginal orgasms often describe them as "deeper" and more satisfying.
The more profound the orgasm, the more likely a woman is to conceive.
We are marvellously engineered, we humans.
(Props to Cyril A. Fox, M.D., "Some Aspects and Implications of Coital Physiology," Journal of Sex and Marital Therapy, v. 2, no. 3, Fall 1976, 209-211).