We only have genitals because we have two sexes: male and female. Sexual reproduction goes back a long way in our evolutionary history.
Many plants and animals reproduce sexually. By convention the male genetic material is mobile and plentiful. The female genetic material is stationary. After fertilisation, the embryonic cells (a combination of egg and sperm cells) divide and grow within the female reproductive part. This standard definition of male and female is used throughout nature. We can see males servicing females in other species of animals. Even insects, such as ants and bees, have workers (males) who service a static female queen.
The male spermatozoa (genetic material) are tiny (relative to the size of the egg) and mobile. There are millions of them (compared with one egg) so they are more dispensable. Sperm typically cover some distance to reach the egg (or ovum). A man’s sperm is produced regularly from his testes throughout his life (until old age). Sperm, together with other glandular secretions, are ejaculated from the penis. This ejaculate is called semen.
The female of any species has a ‘passive’ role. The female genetic material (called ova or megaspores) contains the necessary nutrients for the developing young and so is relatively large. A girl is born with her genetic material: immature eggs that are released (one at a time) from the ovaries into the uterus on a monthly schedule from puberty until sometime after her last period (called the menopause) in late middle age.
Every foetus starts out with the building blocks for both male and female reproductive organs. All embryos (regardless of sex) have Wolffian ducts and Müllerian ducts. These two sets of embryonic ducts determine our ultimate internal reproductive anatomy and consequently our sex. Development of our internal reproductive anatomy depends on hormones produced by the testes (or in the case of the female a lack of hormones). Each foetus ends up with only one set: either male or female. A process called atrophy inhibits the development of the superfluous anatomy.
In a boy the Wolffian ducts develop into the male reproductive organs. The male ducts form the vas deferens (that connects the testes to the penis) as well as other tubes required for ejaculation of semen. In a female these ducts waste away. A man can ejaculate semen, including sperm (male genetic material) because of these glands and tubes that women don’t have. This is clear anatomical evidence that women are incapable of ejaculation.
In a girl the Müllerian ducts develop into the female reproductive organs. The female ducts form the vagina, which leads into the womb (or uterus) where the foetus develops before birth. In a male embryo these Müllerian ducts disappear. This is clear anatomical evidence that the vagina (anatomy that is not present in the male) is not involved in orgasm, which is a response common to both sexes. Male ejaculation is the only justification for orgasm. Female orgasm is a hangover from how the sexes evolved.
Men are lucky. The penis is both a reproductive organ and a sex organ. Unfortunately, the same is not true for the vagina. Men’s sex drive means that they want a partner to be equally enthusiastic about engaging in intercourse. Men ignore the anatomical precedents, which indicate very clearly that orgasm is achieved through anatomy that parallels the male (the clitoris) rather than anatomy that complements it (the vagina). The vagina complements the penis but only for reproductive purposes.
Men can’t have babies because they don’t have women’s reproductive biology. Everyone accepts this logic because men don’t want babies. But men do want women to have orgasms as and when they do. So the logic that women don’t have men’s reproductive biology is not accepted. It is erroneously assumed that women should be able to orgasm from stimulation of their reproductive anatomy (the vagina in particular).
There is a great deal of anatomic and clinical evidence that most of the interior of the vagina is without nerves. A considerable amount of surgery may be performed inside the vagina without need for anesthetics. (Alfred Kinsey 1948)