I love threads as these and the responses it gets. :p
Well, some info from a perv.
The female scent can be intoxicating. Sometimes this scent is transfered onto the panties. If the panties aren't pee/poo stained that only means you get the scent of a woman's private parts. To some men, myself included, that can be arousing. (Though I'd never think of paying for it xD)
Granted, there are those that go further... I've seen guys ask for panties that are heavily poo stained... Kinda gross, but it doesn't harm anyone.
Sniffing panties is quite low on the list of perversions, in my opinion.