I’ve noticed this in several movies and some books. There’s some Awful Influence that destroys people. If it doesn’t kill them outriight, it infects them and they degrade slowly until they die. But the filmmakers want to show this Evil Influence to be somehow seductive, so when it affects the sexiest lady in the bunch, she isn’t affected, or is affected the least, or is affected by still sexy. (An alternative is The Evil Thing isn’t an infecting agent, but some other type of creature or being, but the females unaccountably look like sexy women.)
It;'s almost never sexy guys that seduce the women in the group
This is slightly different from Evil Influences that Take on the Appearance of Sexy Men or Women to seduce people into its clutches, a la the Salt Vampire in Star Trek (which DID appear as male to Uhura), or the Martian plant in Stanley G. Weinbaum’s a Martian Odyssey.
Matango (Attack of the Mushroom People) – obscure offering from Ishiro Honda (Godzilla, et al.) People on a pleasure yachting expedition are driven onto an island (as in Gilligan’s Island), but it’s relatively barren. There’s almost nothing there to eat. Another boat that was driven there is apparently abandoned, but covered in fungus. The island does have edible mushrooms, but if you eat them, you get covered in fungus and start turning into a literal mushroom. People in the stages of metamorphosis look kinda gross, covered in rough green-grey skin, noticeably their faces. When they’ve turned, they look like ludicrous giant ambulatory mushrooms. But when the female movie star (the Japanese Ginger!) eats the mushrooms, her face and most of her body remains free of fungus. She seduces at least one other crew member to eat the mushrooms. You’d think hunger alone would be enough.
Return of the Living Dead --Bad-Girl “Trash” (Linnea Quigley), who dances nude in a graveyard at the beginning of the film, gets repeatedly doused with the Zombie chemical while still alive. When she revives after being surrounded by zombies (and, apparently, killed and not eaten) she revives as a sexy and very active zombie, unlike others who get doused, die, and return as pretty obviously affected undead (like Burt and Ernie – sic)
The Corpse Bride – she’s got a skeletal arm, a PeterLorre-like maggot living in her head, and she’s blue and cold, but she’s got a sexy and attractive shape, full, rich lips, and a lot of skin on her, unlike those skeletonized or rotting Dead she hangs out with, the better to seduce Victor towards drinking “The Wine of Eternity” and joining her in Death.
The Metal Monster – In A. Merritt’s interestingly weird “Lost City” story a valley in the Himalayas is filled with sentient metal beings in geometric shapes that can join together. But the female Norhala, who exercises power over them is very human. It isn’t clear if she’s a corrupted human or an expression of metal sentience in human form.