Vaginal orgasm did not evolve.
Vaginal orgasm is not required. Copulation requires almost no voluntary behavior from women. They just need to tolerate it.
Women do experience clitoral orgasm. Clitoral orgasm is a similarly strong positive effect mistakenly believed to be vaginal orgasm. Clitoral orgasm can be triggered by intercourse, but only if a man’s pubic bone makes contact with a woman’s clitoris. Vaginal pleasure can also help women reach the clitoral orgasm threshold.
The Case of the Female Orgasm, by Elisabeth Lloyd, provides an in-depth exploration of the female orgasm in humans.
Other primates have the same experience as human females: females do not reach orgasm during vaginal stimulation, but do reach orgasm during clitoral stimulation. Primate Sexuality: Comparative Studies of the Prosimians, Monkeys, Apes and Human Beings by Alan Dixson reviews the literature on female orgasm in primates.
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