It's natural for dogs to be really interested in those "close to nature" smells. Heaven's sake! They spend most of their social meetings sniffing other dogs' butts and crotches!
She's fascinated with your scent. We can powder and perfume ourselves, but the crotch region in both men and women carry our natural scents, give away a lot of information about our bodily condition, our strength, our sexual hormonal states, etc. That's something very interesting for a dog. It might sound very strange from a human point of view, but for a dog, it's the essential, physical YOU.
LOL! That said, and good underwear not being cheap, you will have to keep those panties away from her if you want to break her of this habit, and keep your panties in tip-top condition!
Keep them away from her. Make sure any underwear waiting to be washed goes either straight into the washing-machine, or into a closed-lid box. Make sure any clean underwear goes straight into the drawer (she might associate even clean underwear with the habit she has picked up by now)
As for breaking her of this habit, I think that the best way is never to let her do it, never give her the chance. Well, that would be my approach anyway, rather than any other method. I feel she will stop because she won't get the opportunity, and the whole thing will disappear.
My Miniature Pinscher puppy, Duchess (she's 14 months old) does the same thing actually. She likes to chew on them too. We usually sort clothes in one room and keep her out of it. I'm not really sure how to make them stop fully...a vet might know though. I plan on asking our vet when we wind up going back again.
It's not just underwear, it's pads, pants, swimsuits, anything that's been in the crotch region..