The Cherry Myth
The hymen is composed of the same material as the rest of the inner layer of the vagina, a thin, flexible mucous membrane. The tissue has very few nerves or blood vessels. The size of the hymenal opening, thickness, and shape vary from woman to woman. The most common hymen shapes are annular, or circular, and crescentic. Some hymens, known as fimbriated hymens, have multiple projections that look a bit like ruffles or flower petals. Another type, known as a redundant hymen, has so much membrane that it folds back in on itself. Less common is a septate hymen, whose opening is divided by one or more bridges of tissue. In very rare cases, the hymen does stretch across the vagina completely, but this is actually a medical anomaly - called an imperforate hymen - and requires surgery at the onset of menses so that menstrual blood is able to flow out of the vagina. The hymen may be very thin and fragile or very thick and resilient. The shape and thickness of the hymen can change with age. It is sensitive to hormonal changes - becoming more elastic with puberty for example.
Not only is the hymen not what most people think it is, but the majority of women - about 60% - don't bleed their first time. Probably that's why the long list of excuses above for not having an "intact" hymen developed. For those women that do bleed, the bleeding has nothing to do with the special anatomy of virgins but is more likely due to inadequate lubrication and loosening of the vagina because of nervousness (virgins aren't tight because they haven't been stretched out by a penis yet, virgins are tight because having sex the first time can be a nerve-wracking experience), which can lead to the penis creating small tears in the hymen. Those tears usually heal in a day or so, though, so again there's no permanent damage that could reveal under physical examination that a woman has lost her virginity. Some women with very fragile hymens or very small hymen openings may experience permanent tearing or loss of tissue, but women with stronger hymens may not experience any tearing at all. This tearing, furthermore, may not be limited to the first episode of penetrative sex but can happen at any time. It's also possible for tears to occur in other parts of the vagina, meaning the blood may not come from the hymen at all.
It's unclear where the myth started, but there are texts debunking the myth as early as the 3rd century (by the Greek physician Soranus, to be specific), and yet the myth of the hymen as vaginal gateway persists. It has proved incredibly resilient in the face of evidence to the contrary. As I mentioned, most women don't bleed when they have penetrative sex for the first time, and you'd think guys would notice that there is in fact no barrier. But instead of questioning the idea that the vagina is blocked by a membrane that breaks upon penetration, we've invented a number of reasons why the membrane would cease to exist without sexual intercourse. In some cases women who don't bleed may be accused of lying about their virginity, which in certain cultures can be a very dangerous thing. The myth is powerful enough that some women choose to undergo surgery to "repair," their hymens and create the illusion of virginity. These surgeries often involve taking a portion of the vaginal wall and creating a new membrane that will be torn by penetrative sex. In other words, the goal is to simulate the hymen myth, not the actual hymen. So why does the myth have such a strong hold on us? It's true that some women do bleed the first time they have sex, but women can bleed from sex at any time if they aren't sufficiently aroused or the penetration is rough enough. The truth is the cherry myth allows us to think there is a way to tell virginal women from those who've had sex. A woman's virginity is held to be worth much more than a man's, and women are expected to preserve it, in some cultures, until marriage, or at least until meeting someone "special." Being able to prove a woman's virginity therefore is valuable and we've held on to the idea despite the truth. It's long past time we accepted the reality, that there's nothing physically special about a virgin woman, and there shouldn't be anything culturally special either.
(My source for much of the anatomical data is this book.)
Dr. Pisaster has a doctorate in biophysics, not actually anything sexy. She does however enjoy having sex, reading about sex, and talking about sex. Especially when she's had a little whiskey.