Teeth: Tobey tries to rape Dawn. And thanks to her vagina dentata, he gets exactly what he deserves. And maybe a little bit more. Another excellent severed penis prop is just barely glimpsed.
Shivers: This early David Cronenberg movie features a nasty parasite that can only be spread by sexual contact, and in the end, one of the few humans who remains unaffected gets dragged into a swimming pool by the hordes of affected people... to become one of them.
Torchwood: Day One
Another sex parasite takes over hot young people in Cardiff and turns them into sex maniacs... but if you succumb to their sexual invitations, your sexual energy gets absorbed... and there's nothing left of you but a pile of dust. Eep!
They Live. There's nothing worse than when you're right in the middle of having sex with someone, and you suddenly see them for who they really are, with all their faults and shortcomings — like being an evil flayed-skin infiltrator. Ugh.
Nightmare On Elm Street 4: A guy is in bed watching MTV, when he looks over at the sexy swimsuit model poster on his wall. Then, suddenly, the swimsuit model is inside his waterbed, swimming around naked. If only the guy could join her in there. Well, thanks to Freddy Krueger... he can.
Society: Another clip we've featured before, but we have to include it for completeness. It turns out our hero's family are all aliens, and he was adopted. And his family has sex by melting into a mass of seething undifferentiated flesh. Which leads to this part where his grown-up sister's head pops out of his mother and asks if he has any Oedipal fantasies. Urk.
Grim Prairie Tales: A guy gets it on with a woman out in the great outdoors... but then she suddenly starts sucking him inside her as he screams, little by little... until there's nothing left of him. The next day, she's pregnant again. WTF???
The Outer Limits: Alyssa Milano has been taken over by an alien spore (I almost wrote "an alien sore") which compels her to have sex with lots and lots of young men... and then absorb them into herself, so there's nothing left.
Basket Case: Another crazy Frank Henenlotter picture, this time around focusing on a guy and his psychotic conjoined twin. The two of them eventually get surgically separated — but they're still psychically linked, and the twin gets more and more demented. It looks like Duane is finally going to get lucky with the girl he's been in love with for the entire film — but then it turns out his evil twin showed up instead. Urk.
Angel Heart: I'm not even sure what's happening in this Satanic moment from this classic Mickey Rourke film. They're having sex, and then there's blood everywhere, and weird people walking around, and then flames and a bondage orgy. That's just what happens if you have sex with Mickey Rourke.
Liquid Sky: Anyone who has sex with Margaret dies, because aliens are sucking up their endorphins. And eventually the aliens start vaporizing the bodies too. But Adrian bets she can have sex with Margaret and still live. Hence the immortal line, "I bet $300 I can fuck Margaret and not die."
Slither: This weird feminist parable about a husband who literally tries to become his wife's whole world also features these weird penis aliens which jump into the tub while the wife is having a bath. Ewww.
Two teenagers have sex in front of all of the town's children, to propitiate an evil demon that lives in the cornfield and is busy killing Adam from Heroes. You will never be able to get the wrongness of this out of your mind, and yet it aired on the Syfy Channel in prime time.