The things people used to believe about sex were just about as unfounded as the stuff we were told in the back of the bus in middle school. It required very little evidence to turn a Freudian whim into "fact" back then; meanwhile, people were actually told that pleasuring oneself too much would cause hair to grow on the palms of their hands. For real. Sifting through ancient theories on copulation, it becomes entirely clear why sex education has always lagged a bit behind.
Most of these ridiculous rumors were put to bed a long time ago, but a few still pop up now and then. Here's a list of crazy historical theories about making love to casually bring up at your next dinner party.
This holey myth is nothing short of sinful. It was widely believed that Orthodox Jews had to make love while fully covered, to the extent that a sheet was created with a hole in it so the couple could do the deed without any unnecessary exposure. Of course, this is ridiculous. In fact, Judaism considers intercourse a "divine gift" from God, with more functions than just procreation (like pleasure!).
It turns out, this myth may have started as a result of confusion about the Jewish traditional cloth known as a tallit, a square piece of fabric worn over the shoulders with a hole for the head. You can see where the hole thing went off the rails.
(This same false myth has also been ascribed to other religious groups, including Catholics, but it is, in fact, not practiced by anyone, anywhere.)
Because one of society's greatest pastimes is scrutinizing women and their actions - particularly sexually - it isn't that surprising that this myth exists. Freud, the poster child for ridiculous psychoanalytic theories, decided (without providing any evidence, because who needs it?) that clitoral orgasms only occurred in women lacking a certain level of maturity.
When a woman reached puberty, Freud argued, those sensitivities moved to the vagina, and any woman incapable of achieving an orgasm with little to no clitoral stimulation probably suffered some trauma as a child which prevented her from maturing properly.
As you can imagine, women suffered greatly from the rather wide acceptance of Freud's claims and those wacko theories leaked into certain psychiatric practices until the '70s. Yes, the 1970s!
Taboos surrounding menstrual blood are at the heart of most cultures - even the deformation of the Roman god Vulcan (the Greek Hephaestus) was considered a direct result of Juno/Hera and Jupiter/Zeus getting freaky during her "time of the month." According to the book The Curse: A Cultural History of Menstruation, some thought a child conceived during the mother's menstrual cycle was likely to contract a number of diseases including syphilis, virulent ulcers, and leprosy.
This myth is more than inside-out. Galen, an ancient Greek physician, concluded that men and women's genitalia were simply reverse copies of one another. He outlined the different parts of the male reproductive system and found corresponding aspects in a woman's body (the penis was simply the vagina turned inside-out, for example). This mix-up didn't stop him from becoming the most dominant voice in medicine for over 1500 years. Of course, the guy did successfully remove cataracts from his patients' eyes in around 160 AD - so, baby steps.