It’s not a mystery
The female orgasm is considered “one of biology’s greatest mysteries”. I find this view thoroughly puzzling.
I cite and comment below some of the typical reasons given for this supposed mystery.
Just like our digestion, for example, normally takes place without our feeling much of anything, men could have evolved to ejaculate without feeling any pleasure. The ejaculatory mechanism does not require pleasure for it to work. Therefore, just like the female orgasm, the male one has “no obvious purpose besides being pleasurable”. The fact that it happens to be connected to a vital function (ejaculation) does not change this.
Likewise, the fact that the female orgasm may be unconnected to an indispensable procreative mechanism is irrelevant. The sole purpose of both the female and the male orgasm is to give pleasure — nothing mysterious about it.
The male orgasm has never caused much of a stir among evolutionary biologists. The pleasure is precisely linked to ejaculation, the most important step in passing on a male’s genes to the next generation. That pleasure encourages men to deliver more sperm, which is evolutionarily advantageous.
And the pleasure women feel encourages them to engage in sex, which is evolutionarily advantageous. The additional enjoyment that orgasm provides, even if it doesn’t necessarily lead to more children (see below), contributes to the overall well-being of women and to happier relationships—presumably valuable from an evolutionary perspective.
And so could men!
That may be so, but at least that vein of inquiry seems the only sensible one: focusing on how women’s orgasmic pleasure is—by itself—tied to their evolutionary fitness. For example, even if it’s true that women who orgasm more often don’t have more children, it may increase their chances of attracting and keeping a desirable partner, which would be conducive to securing more resources for her children. Or their ability to reach orgasm may help them to gauge, consciously or not, the suitability of a potential long-term partner. At any rate, studying those types of hypotheses would be more reasonable than attempting to explain the female orgasm by looking for a connection with some reproductive mechanism—a connection that doesn’t have to exist for it to have developed through natural selection.