Galen believed that the two sexes were essentially parallel. For him, women were defective men. Using his dissections of animals as evidence (Rome had banned human dissection), he concluded that male and female genitalia were actually exactly the same organs, just inverted. More specifically, he wrote that female reproductive organs were simply defective male reproductive organs that had remained inside the body.**
This defect was a matter of heat. The male body was perfect and had the perfect amount of heat, which the female lacked. It was as a result of this lack that reproductive organs of a female fetus did not emerge from her body when she was in the uterus. The coldness of the female also meant that her testes (ovaries) produced scantier, colder semen, which would, of course, be useless in producing a child. Only the man’s semen contributed to the substance of the child. The woman provided the space for them to grow.
There was another important aspect of the difference in warmth. Galen believed that the reproductive organs on the right side of the body were warmer than those on the left. According to this theory, semen from the right testicle produced a male child, while semen from the left produced a female child.
These days his theories seem patently ridiculous, but for several centuries Galen was cited as one of several authorities on the subject before largely being discredited.
*He’s not the first person to have come up with them, but he wrote about them very clearly and is consequently cited more frequently than anybody else.
**A result of this theory was the idea that if a woman was or became masculine and active enough, her body would make up for the deficiency in warmth, her uterus would fall out from inside her body and she would suddenly become a man.