In this movie, vagina eats you!Teeth is an independently produced black comedy/horror flick about a simple girl whose vagina likes to bite off penises. She discovers her incredible gift one day when her love interest date rapes her. Throughout the movie the audience is treated to pretty much every man in the movie trying to sexually assault the girl, Dawn, and having his penis bitten off as a result. If that makes it sound like it's a Lifetime Movie of the Week with a carnivorous vagina thrown in, that's because it is.
Apparently a child will randomly experience a significant genetic mutation where she will grow giant shark like teeth (they are explicitly compared to the teeth of some carnivorous fish in the film) in her vagina (which is a good 2 feet from where they should grow) AND the musculature that would be necessary for full control of the biting motion of these teeth AND this whole overly complex system developed wholesale, completely intact in one generation without any subsystems developing first and finally functioning together as a whole after generations of mutation. Behold a movie's application of Richard Goldschmidt's "Hopeful Monster" theory, which is a fancy scientific way to say "Yeah, we're totally BSing you here."
Having said that, the film does lampshade how unlikely it would be for this actually to happen. Dawn has a biology class early in the film, and the teacher gives a fairly accurate description of the evolutionary process, mentioning that it's extremely unlikely for a major new bodily function to develop spontaneously. A scene where Dawn looks at the mythology surrounding vagina dentata implies that there was once a time when many women did have teeth down there, but they largely died out after men learned to go after women who weren't likely to bite their penises off during sex, which would presumably make Dawn some sort of evolutionary throwback.
Asshole Victim: All of Dawn's victims, though reversing the usual pattern in that her first victim initially seems a lot more charming and likable before his rapacious lust gets the better of him and her counter-attack is an instinctive reflex, whereas her later victims are progressively more obnoxious and despicable from the start and her revenge on them is correspondingly much more deliberate each time.
Double Standard: Pointed out by Dawn's classmates on about why their anatomy textbook shows the male reproductive organ in all glorious detail while censoring the picture of the female one. Dawn answers that it's because "women have decorum and modesty".
Fingore: The gynecologist learns the hard way that it's a bad idea to try to "test" Dawn's "flexibility" with his hand.
Genre Savvy: For a sick bastard, Brad is clever enough to realize that there's no way that Dawn would actually want to have sex with him, and it takes much persuasion before he finally agrees to go through with it.
Dawn's mother's illness is implied to be a result of this too.
Laser-Guided Karma: At the film's end, Dawn realizes she can deliver this to rapists and perverts.
Lifetime Movie of the Week: Every main male character in the movie (with the exception of her genuinely caring step-father) wants to sexually assault Dawn. In just 85 minutes of screen time, she gets assaulted 8 separate times by 7 different men.
Alternatively, you could look at it as the opposite of Silent Hill, in that its protagonist is a disturbing, psychosexual mutant who has to run a gauntlet of ordinary humans who want to molest her.
Oh Crap: Brad has an almighty instance of this, as he finally realizes just how Dawn severely lacerated one of his fingers when he was a youngster. Said realization comes while he's in the middle of having sex with her.
Sibling Yin-Yang: Dawn is sweet, naive, pure, religious and has a close relationship with her parents. Brad is a moody, rebellious, loud Jerkass who hates and resents his parents. Of course, Not Blood Siblings, but still.
Too Dumb to Live: Dawn runs to her friend Ryan for help and claims that she has murdered people. They have sex. The next day Ryan decides to tell her that she is meaningless to him and he just had a bet to see if he could have sex with her. He tells her this while having sex with her. She then bites off his penis. He might not have believed she had Vagina Dentata, but when a girl claims to be a murderer it's clearly not a good idea to make her angry.
The Unfair Sex: In case it needs to be said again: EVERYONE WANTS TO RAPE DAWN!