People understood the importance of ejaculate for thousands of years. Of course, people wondered how this amazing stuff worked, and ejaculate was among the first things intensely studied by Anton Van Leeuwenhoek, who is credited with inventing the first microscope.

This led to the discovery of sperm, and a subsequent debate about whether sperm are really the origin of the embryo, rather than the egg.

A nice discussion of the topic is here