Sexology, or the study of human sexuality, is a science at the nexus of biology, neurology, psychology and sociology. I had no idea it existed, but after some research, I found out a good amount. And like any science, sexology has its eureka moments. Here are some of the biggest, roughly in chronological order.
Non-procreative sexual behavior is common
In 1886, a psychiatrist named Richard von Krafft-Ebing revolutionized the discipline of sexology by publishing his exhaustively researched tome Psychopathia Sexualis. He'd documented every case he could find of what he called "sexual perversity," including those he'd encountered first-hand among his patients. He defined sexual perversity as pretty much anything that deviated from procreative, heterosexual sex, and put each perversion into its own special category. Though he intended to document perversity, the book had the opposite effect: Many doctors and ordinary people read it and realized that many kinds of "perversity" were so common that they were almost normal. The (relatively) unbiased reporting and taxonomic structure of Krafft-Ebing's book inspired countless other early-twentieth-century researchers, including Sigmund Freud, Magnus Hirschfeld, and Alfred Kinsey. Though published over a century ago, Psychopathia still has the power to shock.
Bisexuality exists
Sigmund Freud, the father of modern psychiatry, is famous for remarking that everyone is bisexual. His idea was remarkable for two reasons. One, it acknowledged that there was a middle position between gay and straight (a relatively rare belief among doctors); and two, it paved the way for a more nuanced understanding of how sexuality exists on a continuum rather than as a binary system. Jumping off from Freud's idea, infamous twentieth century sex researcher Alfred Kinsey created what has come to be known as the Kinsey Scale for sexual orientation. On that scale, 0 is completely heterosexual and 6 is completely homosexual. Kinsey and his colleagues did decades of in-depth research to determine that most people fall somewhere in the middle of the scale. You can see their research in Kinsey's most famous works: Sexual Behavior in the Human Male and Sexual Behavior in the Human Female. All research was based on thousands of anonymous interviews conducted all over the United States.
Medical science can transform men into women, and vice versa
Throughout recorded history, there have been women who lived as men and vice versa. Many cultures even have the idea of a "third sex," often a shamanistic role, which is for people who are neither male nor female. But it wasn't until 1930 that the first sex change operation was performed on a famous Dutch artist named Einar Wegener, who emerged as the woman Lili Elbe. Unfortunately, the operation was crude - it involved implanting ovaries - and she eventually sickened and died (you can read her intriguing memoirs about her transition). The first successful male-to-female sex change operation was performed in Denmark in 1952, and its recipient, Christine Jorgensen, became an international celebrity. Since then, thousands of people have had successful sex reassignment surgeries, moving from female to male and male to female with the assistance of medical science.
Women have orgasms
The female orgasm has been "discovered" several times over the past 130 years. In the nineteenth century, doctors used vibrators to help relieve women of "hysteria." though almost no medical accounts from the time acknowledge that this therapy was basically masturbation. The Victorian Era had given rise to the myth that women didn't have orgasms, and many medical researchers adopted this idea as truth because it was impossible to prove that women were orgasming the way you could prove men were. Though anecdotal reports and throughout the twentieth century indicated women could orgasm the way men could, it wasn't until the experiments of sexologists William Masters and Virginia Johnson in the late 1950s that the female orgasm was finally proven to exist in a scientific manner. Masters and Johnson observed women in the process of orgasming while monitoring everything from blood flow to muscle spasms in their vaginas. (Yes, they actually inserted a dildo-shaped measuring device into the women's vaginas to do their research.) After Masters and Johnson published their research in 1966, several other researchers investigated women's sexual response cycle, quickly discovering the G-spot, female ejaculation, and even looking at orgasming women in MRI machines (the picture of that at the top of this post). Recent research into female orgasm has focused on the neurochemistry of women's brains while they are aroused.
Pregnancy can be prevented with a pill
In 1960, the birth control pill debuted on the market as a contraceptive for women. In the late 50s it had been prescribed to women who suffered extreme menstrual cramps. But in the early 60s the pharmaceutical known as "the Pill" became not just a sexology discovery but shorthand for a sexual revolution that had more to do with culture than science. Freed from cumbersome birth control devices like condoms that depended on male cooperation, women could suddenly have sex without the constant worry that they would become pregnant. Many historians have argued that the Pill helped start a new wave of feminist consciousness. The Pill is an excellent example of how a scientific discovery can have widespread, unintended social consequences.
Homosexuality is not a disease
In 1973, homosexuality was removed from the Diagnostic and Statistical Manual of Mental Disorders (DSM-II). That meant that after decades of debate, the professional psychiatric community would no longer treat homosexuality as a disease. Certainly an unhappy homosexual might be viewed as neurotic, but a happy, well-adjusted gay person would be given a clean bill of health. Many sexologists had been arguing for decades that homosexuality was not a disease, most notably the openly gay psychiatrist Magnus Hirshfeld, who founded the Berlin Institute for Sexology (which was later burned down by the Nazis). But the removal of homosexuality from the DSM made it official: Licensed doctors now agreed that gayness on its own was not an illness.
Many kinds of male impotence can be cured with a pill
In 1998, men got their own version of the Pill. A chemical called sildenafil citrate came to market under the name Viagra. Sildenafil works by relaxing muscle tissues, allowing more blood to flow into the penis. Just as the Pill liberated women from fears of pregnancy, Viagra liberated many men from fears of impotence. While sildenafil didn't set off a cultural revolution, it did represent a major scientific breakthrough - and has helped researchers understand male sexuality better. Viagra and similar drugs like cialis are among the bestselling "lifestyle pharmaceuticals" of all time, raking over 1.5 billion dollars per year.
Orgasms can be caused via direct neural stimulation of the spinal cord
In 1998, the same year Viagra hit the market, Dr. Stuart Meloy made a strange discovery while operating on a woman's spinal cord. He was stimulating her nerves in order to locate the source of her back pain, and when he hit one particular nerve he gave her an instant orgasm. "You should teach my husband to do that," she told him. Meloy went on to patent a spinal implant device, which he hopes to market as a cure for female sexual dysfunction (i.e., an inability to have orgasms). He's in the process of testing the device now, and is actively seeking volunteers - female and male - so that he can perfect the device and bring it to market. Once he's got a version of the device that people can use easily, you can expect a sexual revolution that will make the Pill look like a walk in the park.
Women ovulate more than once per month
In 2003, a researcher named Roger Pierson at the University of Saskatchewan overturned the almost century-old scientific belief that women ovulate once a month. He and his team used simple ultrasound scans on 63 women with normal menstrual cycles, and discovered that a significant number of them ovulated 2 or 3 times per month. Their finding could have a significant impact on how we understand female hormonal cycles and fertility.
This one requires some backstory - First off, The Dude in Big Lebowski was inspired by a real guy, Jeff Dowd. Jeff Dowd was recently working as a producer’s rep for Dirt! The Movie, a documentary playing at Sundance this year. Following a screening, Dowd was discussing the film with Variety film critic John Anderson, when both of them exhibited some very un-Dude-like behavior: 
