Long before we are born, the same gonads either become testes, if the embryo has a Y chromosome, or ovaries if it doesn’t. Once that process starts, other changes happen that tell the testes to make sperm and the ovaries to make eggs. There are some rare cases where only one of the gonads gets the message from the Y chromosome, or only part of the gonads gets that message. In that case, an individual can make both eggs and sperm.

Some of the reproductive structures start out the same in both males and females, but become very different when the testes or ovaries start making hormones.

That is essentially what makes women different from men: women produce eggs, men produce sperm. Why humans have separate sexes and why we're not all hermaphrodites (everybody having both male and female sex organs) is an important evolutionary question that has not been adequately answered.

Women don't produce sperm because they do not have the Y chromosome, which possess a gene that tells the body to produce male features. Humans, like all mammals, have two separate sexes. Most animals have two different sized games, where one sex produces a few large egg and the other sex produces lots and lots of smaller sperm. The theory is that this is to maximize the nutrients the young start with, the large energy rich egg, and the likelihood that the egg will get fertilized, using lots of sperm to ensure one makes it.

If animals have the necessary genes to produce either of the gametes, they could produce both. There are some other animals that are capable of producing eggs and sperm (hermophrodites) and other animals that will change from producing eggs to producing sperm, or vice versa. Animals such as slugs and worms are hermaphrodites. And there are some species of fish that will effectively change sex, starting off producing eggs, and then when the dominant male in the group dies, a female will change to produce sperm to replace that male.

Women can’t produce sperm because they produce eggs. Sperm fertilize eggs, and this is what creates the fetus, which will grow and eventually become a baby. If women produced sperm in addition to eggs, the sperm would fertilize the eggs and women would constantly be producing babies.

Eggs contain half of the genetic code (chromosomes) to make a baby. Sperm contains the other half of the code, and when the egg is fertilized, there is enough genetic code to produce a fetus.

The human reproductive cycle has differentiated sexual organs and functions for the two biological genders. However, the sexual organs of men and women are highly analogous in both structure and function, even if they appear to be different. For example, each gender produces gametes (cells specifically for reproduction, with only one set of chromosomes) in specialized organs that are paired: the ovaries produce eggs in women, and the testes produce sperm in men. Genetic material from both an egg and a sperm are required for the combined gametes to begin growing into a new human being.

Women don’t produce “sperm” because we’ve defined that term as male gametes. The differences between the gametes arise from the division of responsibilities in our reproductive cycle. Since females nurture the combined gametes into a fetus in a womb, the eggs more closely resemble other body cells in size and function, e.g. they have mitochondria and other organelles. Sperm must survive the transfer from male penis to female uterus through the vagina, and thus have changed to make the process easier. Sperm have a “tail” that allows them to move through fluids, sacrificing internal structure. This change makes their lifespans shorter than eggs,so sperm need to be made more often – unlike eggs, which are made early in a woman’s life and cannot be made as needed.

These sex differences have been a successful strategy for our ancestors and for other animal species, but it’s not the only way to divide up the work of reproduction. For example, not all females nurture their fetuses during development (pregnancy) – despite contributing sperm-like gametes, male seahorses carry the fertilized eggs in a womb-like pouch until they hatch. Clown fish and several species of frog can change genders from female to male if necessary, switching from producing eggs to sperm. Many flowering plants have both male and female sex organs, either within the same flower or in separate flowers on the same branch, distributing spores to other plants (“sperm”) and accepting spores to form seeds/nuts (“eggs”).

There is great variation in the anatomy, gender and reproduction cycles across animal life – in that context, the sex differences between male and female in our species are both arbitrary and fascinatingly complex. Stay curious!

Women don't need to produce sperm. Women produce eggs and nurture the growing embryos. That's just the way life is. With 2 sexes, there is more variety in the offspring, because each offspring gets half its genes from each parent, so each offspring is a unique individual. With more variety in the offspring, the species is more resilient to changes, because some offspring are able to survive best in some conditions and some are best able to survive in other conditions.