Abstinence

Not having sex is a "voluntary control over the discharge of semen." It is also a "natural and foolproof form of birth control and eliminate[s] the occurrence of unwanted conceptions." If you don't have sex, you won't have kids.

You can you go two ways, I think.

Way 1:

Sperm production follow a periodic cycle, roughly like egg production in the female does. This means that only intercourse during the right moment will be fertile, any other time will be non fertile. To prevent extinction of course you need a synchronization of female and male periods.

Way 2:

The male has to consciously decide for emitting semen, something along the line of the western interpretation of tantric sex. A muscular contraction at the right moment can simply stop the seminal emission.

Retrograde ejaculation

Retrograde ejaculation occurs when semen which would, in most cases,
be ejaculated via the urethra is redirected to the urinary bladder.
Normally, the sphincter of the bladder contracts before ejaculation
forcing the semen to exit via the urethra, the path of least
resistance. When the bladder sphincter does not function properly,
retrograde ejaculation may occur. It can also be induced deliberately
by a male as a primitive form of male birth control (known as coitus
saxonicus) or as part of certain alternative medicine practices.

You want some deal where the men can fulfill their culturo-evolutionary imperative to have lots of sex, but shortcircuit the fitness benefit that it historically provided. That will work until mutant men who can't do the short-circuit overwhelm the population with their progeny.

But in the short term, your men can be yogi masters (if that was not already the case) with conscious control over the bladder sphincter. Sex occurs normally, with the ejaculate captured in the bladder to be voided against a tree at some later time. Out of distaste for banality, I hope you make these men and their partners be capable of all sorts of other yoga sex tricks too.

Intelligence has allowed mankind to thrive, and your humanoids might have evolved it out of a necessity.

First option: disease. We already have a common disease that has a ludicrous chance to kill an unborn child. Biology found a solution: kissing. By kissing you exchange saliva and if the man has the disease he'll infect the woman. By the time the disease becomes dangerous to the baby the woman has created an immune response and the baby is safe. Now imagine a disease that primarily lives in the reproductive organs and the man will have an idea when he's infected during sex. The disease works like the flu and changes each year, so you dont stay immune. If the disease survives long enough to have a high chance of killing the child the man can choose not to ejaculate but continue sex normally to both increase the chance of infecting the woman and give her time to become immune and for the social benefits of sex.

The second and more elaborate option: Imagine a world where there are large stretches of time where unborn children and babies have almost 100% chance of dying. Its a waste of time, energy and potential children as you need to recover after a failed attempt. If these stretches of time are happening at predictable intervals biology would simply adapt the cycles to those intervals. If they arent predictable only two solutions remain: Make sure your biology can create children at a rapid pace or become intelligent enough to learn all the signs when children wont be viable. For potenial comments like "biology would learn the signs of the event and not need intelligence" there would need to be several events that seperately may not cause death in the babies but combined would cause that death.

Sex would still be used as a social interaction between the man and woman in those times children shouldnt be born, but to ensure no babies are born the man would prevent ejaculation.

We humans are capable of doing it.

Millenia ago the people in India developed some traditiond which are called Tantra. Among such traditions, there is tantric sex. Search on Google or Amazon... There are thousands of books on it written just this century.

Central to tantric sex for men is developing the ability to have dry orgasms without being dry. This is done through breathing and muscle control, akin to (or including, depending on the view) yoga practices. It is said to become second nature once you achieve it, and allows for a man to have multiple orgasms.

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.

1. 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.




2. 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.




3. 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!!!

The testes have a muscle called the cremaster, that pulls them up whenever it's too cold to maintain the right temperature for spermatogenesis (the process which sperm cell are produced), or relaxes to cool the testes whenever it's too hot. Have this process be a lot more temperature dependent, working only when the testes are around the temperature of the body.

The cremaster muscle should not be an independent muscle, or involuntary. It should be a voluntary act to contract or relax it, depending if you want to be fertile or not. In this system, you need to pull the testes in advance, maybe some hours prior to sex, in order to have functioning sperm cells.

Although you'll have no chance of conception, semen will be produced normally, as the majority of its components is not produced in the testes, but on different locations of male genital tract.

These are humanoids so we don't have to modify human men.

In their society males, like seahorses raise young.

Male seahorse giving birth https://youtu.be/MsHCqrrU-Gk

This is very energy expensive so males don't want to be having offspring all the time.

However females will wander if they don't get regular sex. Therefore, through evolution, the males have a mechanism for ejaculating only when their previous brood is self-sufficient.

Look to the bladder.

The male urethra has two purposes: the ejaculation of semen and the elimination of urine. For most men, these two functions are mutually exclusive; it is difficult to maintain an erection with an extremely full bladder, and urination becomes very difficult when a man has an erection.

Brief anatomy lesson: the urinary and reproductive tracts meet inside the prostate gland, which surrounds the urethra and rests directly below the bladder. The vas deferens carry sperm from the testicles to the prostate, where they merge with the seminal vesicles and join with the urethra via the ejaculatory ducts.

Downstream of the prostate lies the smaller bulbourethral gland. Human urine is acidic and poses a threat to the health of sperm. The bulbourethral gland secretes a slightly alkaline fluid, often colloquially referred to as "precum", which neutralizes the urethra prior to ejaculation. Semen itself is also slightly alkaline in order to neutralize the natural acidity of the vagina.

I can think of two ways that we utilize the confluence of these two systems to achieve the desired effect:

1. Males of your species are capable of urinating and ejaculating simultaneously. If the urine of your species is sufficiently acidic, it will kill the sperm before or shortly after they leave the penis.


2. Males cannot urinate and ejaculate simultaneously, but can choose to urinate at any time and the act of urinating forcibly closes the ejaculatory ducts. In my opinion, this is the more reliable and realistic solution, since the closing of the ducts would prevent urine from entering the vas deferens where it might damage future sperm. However, the sperm would probably leak out afterwards, so caution would need to be exercised in order for this to be truly effective.