3. An Alien
As seen in: Species (1995)
As bizarre or disturbing as some of our list-makers' choices in sex partners have been, for the most part, the copulating parties knew what they were signing up for. What you see is what you get, right? Not if you're messing with Sil (Natasha Henstridge), an escaped killer alien with an extreme case of a ticking biological clock.
Posing as a sexy blonde, her goal is to find mates and reproduce as soon as possible before she's tracked down by the scientists that recognized her capacity for harm in their labs. What lamely wasn't recognized, more specifically by one of her trackers, Dr. Stephen Arden (Alfred Molina), was a simple change of hairstyle. After getting busy with the woman he's spent the course of the movie hunting, the pillow talk is put on hold for his love to sprout tentacles and (presumably, judging from that last shot) stab him in the face with her razor tongue, as she's prone to do.
Dudes, let this be a lesson. Be sure to notice when your lady updates her look. If you don't, chances are it'll come back to bite you in the ass later.