Proceptive (?) medication - Males have to take a drug to produce sperm/gametes or one of the other necessary components of semen.
I'm envisioning something plant- or fungus-based...
Your early hominids live in an area where this organism is plentiful--so plentiful that they never really have to think about actively producing sperm. Some males don't like the taste, so they rarely eat it and maybe never have children or have very few. Groups that splinter off and leave the home area only reproduce if they move to an area where the same plant grows or if they take the dried fruit with them. Maybe they aren't consciously aware of it, but there's definitely a connection between the fruit and reproduction. They might even have a sex/harvest festival when the fruit ripens.
Over time, as agriculture improves and your hominids explore their planet, their diet becomes more varied, the sperm fruit is less of a staple, and the birthrate slows down. It has become a cultural tradition for explorers to always carry a potted sperm plant with them. Sailors have been known to mutiny and force their captains to turn home whenever the potted plant dies. Horticulturalists attempt to create sperm fruit hybrids and succeed, introducing more variety into both the plant and the hominid species.
Perhaps the sperm fruit plant is very climate-sensitive. That could be a form of natural population control. In lean times, the plant and the population don't grow as much, so there are fewer new mouths to feed. And it could lead to developing trade between groups that live in areas where the plant will grow and the areas where it won't. This sets up a few different scenarios.
A global climate catastrophe wipes out 90% of the world's population. Maybe only half of that is wiped out by the actual disaster. The rest simply live their lives and die without the ability to reproduce. Only those isolated areas where the plant can grow have thriving populations. This could lead to development of cultural variations.
Politics vs. Science - Your hominids have established a clear link between the sperm fruit and reproduction. Now there's a race to create a shelf-stable synthetic version so that they're no longer dependent on the fruit. The government tries to control growth and distribution of the sperm fruit, causing groups of rebels to periodically attempt to steal the plants and/or overthrow the government.
Space drama - Your scientists haven't yet succeeded in developing alternatives to the sperm fruit. Or they have, but the synthetic version has a short shelf life/is costly to produce. Space colonists must take living sperm fruit plants (either in stasis or specialized terrariums) with them on their colony ships. An alien attacks! A meteor strikes the ship! All but one of the plants are destroyed, and that one is damaged! What will they do? Will the colony survive? The ships's horticulturalist was captured in the attack--will they be able to get him back? Can his daughter, who's angry at being ripped away from all her friend back home, decode his notes and heal the plant? Watch the drama unfold!!!