An enzyme, enough of it, macroscopically would look like some kind of goop?
They used to be primates (maybe) with no culture or language and continued to make babies by accident without any cohered effort.
The vissians naturally gravitated towards threesomes (upwards to full village orgies?) where the three sexes participated before the vissian cavemen could tell the difference between a woman and a congenitor, unless once naked, the difference is glaringly obvious, since how do you throw goop into a cavewoman's vagina with a different cavewoman's vagina?
1. Congenitors have a launcher genital that throws goop into women. You remember He-Man and Skeltor (toys in the 80s) had a sword each which were actually two halves of the same sword, that snapped together, and could be used as a key to open the gates to Castle Grey Skull? Maybe the congenitor genital and the penis interlock into a unified super penis before it intrudes into a vagina, and then it's like the dessert cart at Pizza hut, that lets you crank several sauces allover your ice cream at the same time.
2. Congenitors have a receptacle where like an old timey quill and inkwell, the penis reaches into the conginitors receptacle, draws up the goop into itself like a momma bird getting ready to feed her chicks, and takes the congenitor goop to a near by insemenatable vagina. That would mean that a vissian penis has two settings: Suck and Blow. Way too complex, cavemen would not be able to figure it out and they would have died off millions of years ago.
3. It's all in the sweat. A little heavy petty, and every one is fertile.
4. It's a sneaky lie. Overpopulation. No one needs a congenitor, but their government is lying to them, and they are falling for it.
5. It's a political decision underpinned by Eugenics, so that they can deal with serious over population issues. Therefore cavepeople never had to have had to figure out three sexes sex to perpetuate the species. It's new. This option would also suggest that congenitors are made, and they are not some random effect generated by the population. It also means that if the Vissians drop their population to a preagreed upon number, that they can dispense with congenitors, and go back to trusting idiots to breed responsibly.