I'm not one into some of the variety of stains that can appear on underwear but one way that panties can be "dirty" and erotic is if the panties were wet from arousal.
Say a woman had been watching a movie that got her very turned on prior to the rapist attacking her. Or if she got wet against her will from the build up to the actual rape.
If her panties are moist from her arousal then definitely using them as a gag would be an erotic humiliation.
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