Back in biblical days, scholars were able to recognize fertility rates in conjunction with sexual activity. They didn't understand why, but they found that men who didn't masturbate and only had sex so often had more children than the sexual deviants. Prior to modern medicine, birth rates needed to compete against mortality rates in order to maintain civilization.

Do not use modern society as a basis for old society's reasoning. It all sounds bizarre, but education was not as evolved as is today. Further, modern society in America is contaminated with Puritan influences.

I suggest researching Sigmund Freud. Why? Freud is responsible for the term "anal retentive" or what we commonly refer to, "asshole". He theorized that strict potty training as a child resulted in people becoming assholes as adults. Sigmund Freud is an Austrian born in the mid 1800s. Reviewing the data behind Freud's theory on the "anal retentive" should answer all of your questions on such studies.

In old science, you didn't need experiments and evidence to be a scientist and make such claims. You just needed the renown and someone to print it. The concept of "the vagina will absorb more sperm after an orgasm" is someone's observation of an event. Really, sperm don't survive well outside of the testes, and the seminal fluid helps sperm survive the trip. Having the female orgasm also supplies more similar fluids from the female to improve sperm survival. (Yes, it only takes 1 sperm to fertilize an egg and the female doesn't have to orgasm. The point here is improving odds of fertilization, as many sperm will die on the trip.) You can say the scientists of the time saw the data, whatever it was, and made up this conclusion because this made the most sense to their common speak and religion, how they understand reality.

Think of these studies as stepping stones into modern science. Yes, we have "weird" studies like this today because we have young and eccentric researchers looking for some study that hasn't been done many times before.