You know, Dawn's first victim didn't exactly start out planning to rape her... but then, it's not as if she ever intended to bite his dick off either. This means her retribution against him is a particularly poetic kind of justice; Laser-Guided Karma indeed!
Despite how many dismembered members the movie is willing to show in all their gorey glory, the titular Dentata'd vagina is never shown on screen, even as a diagram or x-ray to show how it works. This is technically lampshaded by the movie itself in the sex ed class scene: people are seemingly more comfortable seeing dicks than vaginas ("It's different", somehow), so of course horrifically mutilated penises will be shown en masse, but the vagina that severed them, despite being the main point of the movie? God forbid anyone sees that. It has the ADDED bonus of showing why that kind of censorship is harmful: because no one ever really got to see it/its function, now no one, even/especially Dawn, will ever truly understand how her privates work, beyond naive intuition (and... practice/reflexes on Dawn's part, I suppose).
Hey, one guy didn't get castrated the first time he had sex with Dawn; it was the next day when she realized he didn't care about her at all and was just using her that he paid the price. What this means? Those teeth are retractable in addition to all the other improbable Required Secondary Powers she's developed. Stephen Jay Gould, call your office!
That Dirty Old Man at the end? Judging by his behavior in the final scene, the particular kind of sexual assault he's got planned for Dawn is going to cost him his tongue.
[Arguably, this crosses with Horror.] One additional possibility for why, positing (for the sake of argument) that the Vagina Dentata has occurred at any point in the genetic history of homo sapiens, nature selected against it: in order to pass on genes, you need to reproduce; meaning that somebody with a vagina which is weaponized and semi-independently tries to bite the hell out of anything living when its owner is sufficiently upset is about to be called upon to pass a very large object through a birth canal lined with teeth under the traumatic condition of suffering labor pains. How likely is the baby to survive such a birth so he/she can carry on the genes?
Then again, if the teeth are retractable and the baby's mother does have control, the genes do stand a fairly good chance of being passed on. At the same time, the prospective mother is likely to have some difficulty landing a man to father her child in the first place if she gets a reputation for biting.
Not necessarily—childbirth is, shall we say, uncomfortable, and women don't necessarily have a lot of physical control over themselves during the process. Lots of women poop themselves while giving birth. It seems that even if the teeth are retractable, they might accidentally come back out during the trauma of birth.
Regardless of whether or not they're retractable, a Caesarean Section would still probably be the best option for her to give birth.
In view of the above reproductive problems, one possible explanation for how women equipped with the Vagina Dentata manage to propagate their genes? One of their Required Secondary Powers might involve exuding some kind of pheromone that makes them sexually irresistible; which in turn might explain why all the guys in this movie are after Dawn and none of them willing to take "No!" for an answer.
When you realize that despite them throwing it about as a defense for rape, Dawn gets raped every-time. She may as well just castrate her rapists by hand afterwards. The ending scene with the old man where she smiles as she prepares for him TO RAPE HER is especially disturbing, as it implies she's come to terms with the idea of being repeatedly raped throughout her life and is just glad she can bite their dicks off once they get busy. Yikes.
Even worse, she ends up with a vagina full of blood each time. While she always bites before the risk of a Child by Rape becomes an issue, the risk of STD's is going to be high.
Alternatively, the fact that she's preparing to allow the old man to have sex with her means that she now derives pleasure from biting penises off. In other words, the act of biting is her orgasm.
Third possibility: she's taking up a new career as Castration Woman, the heroine with an unlikely superpower, entirely of her own free will and is just smiling because she didn't expect a chance to ply her new trade so soon.
