Incidentally, the above mythology went a long way to explaining away pregnant nuns in the Middle Ages without destroying their virtues.Better to be seen as the victims of demonic rape than participants in consensual sex, one might suppose.
When paired with Beauty Equals Goodness, this is reconciled by making the good characters' beauty more genuine and the evil characters' appearance more focused on inspiring lust, separating "pure" beauty from sex appeal.
Thus, the hot chick likely was Evil All Along, or has made a Face Heel Turn; the constant heroine, although beautiful, is not sexy because she exudes Incorruptible Pure Pureness even if she's going to end up a wife and mother. Also sets the stage for Unfortunate Implications, frequently invoking various Double Standards attached to the sexuality (and the "goodness" of that sexuality) of a character depending on their gender.
This is mostly applied to female villains, although attractive male villains are becoming more common, especially in series geared toward a female audience.
This character's almost guaranteed to garner loads of entirely undeserved sympathy from fans (especially if male), but is so ingrained in certain cultures that it's not going away any time soon.
(Okay, maybe that last Pot Hole went too far.) Their outfits tend to show more skin and have more leather, fetishist hangups, lace, on occasion, or even gorgeous and functional Femme Fatalons.