Since medical school began, I've only managed to read one non-required book in the last two months. It's not a fact I'm proud of but I think I spend my scarce time wisely. I read Hanne Blank's Virgin: The Untouched History, an account of the history and nature of virginity. It's an accessible and entertaining read for my overtaxed brain.
This was one thing I learned:
Despite the lack of any actual studies in the literature regarding whether horseback riding, gymnastics, or riding bicycles might have to do with womens' hymens, virtually every contemporary writing about virginity aimed at teen girls is duly equipped with a disclaimer that says something along the lines of 'many girls tear or otherwise dilate their hymen while participating in sports like bicycling, horseback riding, or gymnastics.'"Woah. There is no scientific evidence that these activities stretch or tear the hymen! Yet I've heard this countless times in teenage girl magazines or otherwise informative literature on puberty and sexuality. Be sure, this "fact" is not just something from conservative abstinence-only sex education curriculum but widely seem in popular and generally accurate sex ed. It's probably in those puberty books your pediatrician recommended you to read. Understandably this belief was popularized in order to dissociate hymen with virginity. In recent years (decades?), it's become more acceptable for girls to participate in sports and the hymen less a gauge of virginity.
I think it also shows that the empirical evidence or lack thereof don't affect people's beliefs that much, in sexual matters and otherwise. In medical school we grumble all the time about evidence-based medicine. It should dictate medical practice but often it doesn't. Doctors and patients often want and perform procedures that aren't medically better than the other options.
Have you heard this when you were growing up?