I was reading about the 12th century Christian sect catharism on Wikipedia, which claimed that the group refused to eat anything that was a product of procreation (meat, eggs, milk), but that they ate fish because the thought of the time was that fish naturally came about by spontaneous occurrence. Is this accurate? If so, when did we discover that female fish lay eggs and that male fish fertilize them?
Since no-one has responded yet, I'll try my hand at an answer. Hopefully it is up to this sub's standards.
I can't say much about your specific question about fish, but I can talk about the general idea. The idea that living things can spontaneously arise from non-living things is called spontaneous generation, and it was a widely-held scientific theory until the 19th century. You mention the idea that fish didn't procreate, but spontaneously generated in the seas. Other examples would be the idea that maggots could arise from dead flesh, or mice could arise from haystacks. The core idea is that, while some forms of life can only emerge from parents, other forms of life can come from non-life without parents, seeds or eggs.
This theory was discussed by some ancient Greek philosophers, among who Aristotle, who wrote in his History of Animals, "some [animals] spring from parent animals according to their kind, whilst others grow spontaneously and not from kindred stock; and of these instances of spontaneous generation some come from putrefying earth or vegetable matter, as is the case with a number of insects, while others are spontaneously generated in the inside of animals out of the secretions of their several organs." Making a big jump in time, in the 17th century Jan Baptist van Helmont performed some experiments on the origin of life. Among his notes, he writes 'recipes' for mice (soiled cloth and wheat) and scorpions (put basil between two bricks, and leave it in sunlight).
From then on, the theory of spontaneous generation was challenged more and more. I'll give some examples of experiments. In 1668, Francisco Redi studied maggots spawning from rotting meat. He found that maggots 'generated' from meat in an open jar, but not from jars covered in a fine cloth. In 1768, Lazzaro Spallanzani boiled broth in a sealed container to kill all life in the broth, and then observed whether there was any spontaneous growth of life. As microscopy developed and scientists learned more and more about the origin of life, the theory of spontaneous generation slowly came to an end.