Human sexual anatomy and development

Sexuality is about appreciating the human body and sexual anatomy. We should understand the function of the male and female genitalia as well as the changes that occur throughout our lives. It is also useful to know what kind of stimulation may be pleasurable or is involved in achieving orgasm.

By combining gene pools from two parents, sexual reproduction allows more rapid development of genetic diversity, allowing a lifeform to adapt to changing environments. The step of evolving two sexes was inevitable because of the huge advantage that sexual reproduction provides. Those individuals with a more advantageous set of genes reproduce and survive more successfully (on average) than those who are less advantaged. This evolutionary process of life on earth is called natural selection.

There is a specific chromosome that determines our sex. A man has an XY chromosome and a woman has an XX chromosome. There are other combinations (involving three chromosomes) but the Y chromosome is associated with being male. A boy inherits a Y chromosome from his father. The father’s genes (not the mother’s) determine the sex of a baby.

Every human foetus has a pair of gonads. The chromosomes determine whether the gonads in the embryo become the testes (for a boy) or the ovaries (for a girl). The gonads become the centres for production of genetic material (eggs or spermatozoa). The ovaries store the eggs, which are held until they mature and released one at a time during a woman’s monthly cycle. Once the testes of a male foetus start producing androgens (primarily male hormones such as testosterone) they drive further change, which is concluded at puberty. This process is called sexual differentiation.

We can see the evidence for how orgasm evolved by looking at development of the human foetus. Every foetus has a rudimentary phallus. This is called the genital tubercle. It is impossible to determine the sex of a foetus until after 14 weeks. Only the extent of the development of the phallus varies depending on hormones sent out by the testes (or not). Unlike the internal reproductive anatomy, the phallus is not an either-or option. Everyone has a sex organ and so everyone (to varying degrees) is potentially capable of orgasm. The genital tubercle undergoes significantly more development in a boy so the penis is larger than the clitoral organ.

Orgasm is a primitive response of the human body that has evolved to trigger male orgasm. Even though female orgasm is much less common, it is achieved via an identical mechanism. The phallus is the only organ that is involved in orgasmic response. The penis and clitoris are easily reached by the hands so that self-stimulation (masturbation) is possible. This enables a responsive individual to supply their own stimulation if desired.

We all achieve orgasm through the same mechanism regardless of gender and orientation. Women’s genitalia (clitoris and labia) are anatomically equivalent to men’s genitalia (penis and testicles) and are similarly positioned. The labioscrotal folds of the embryo become the scrotum (for a boy) and the labia (for a girl). As a woman ages, the parallel with the male equivalent becomes much more apparent as the skin either side of her labia majora looks increasingly like the wrinkly hairy skin of the testicles.

Sometimes we have only one body part. We have one head and one sex organ. Sometimes we have two for backup. We have two arms and legs. There are relatively few anatomical differences between the sexes. A man has nipples but he never develops the mammary glands that form the female breast. The key differences relate to sexual anatomy but even here there is symmetry. A woman has two ovaries and a man has two testes.

Girls and boys have equal amounts of androgens until age 7 or 8. During adolescence the levels of androgens rise markedly in boys but less so in girls. Women have around two thirds of men’s levels of androgens, including testosterone which is involved in libido and sexual response.

Pre-adolescent boys and girls have equal amounts of estrogens. During adolescence estrogens increase abruptly in girls but there is only a slight increase in boy’s levels. Women have much higher levels of estrogens than men have. There is a steep increase in male responsiveness at adolescence but a much reduced and slower increase in female responsiveness. Female responsiveness is evidenced by a woman’s solitary masturbatory activity. A responsive woman generates her arousal consciously by focusing on eroticism. As a result of clitoral tumescence (increased blood-flow), she instinctively stimulates the internal clitoral organ and thrusts to orgasm.

… the reported levels of the 17-ketosteroids in the human male differ from the reported levels of the 17-ketosteroids in the human female in a manner which more or less parallels the differences which we have found between the median frequencies of orgasm at various ages in the two sexes. (Alfred Kinsey 1953)