It's not the worst anti-rape device I've ever heard proposed (that would be the spiky tampon... ouch) but it does seem like a bit of a depressing and restrictive piece of kit.
Scenario 1:
"Thanks for the lovely evening... say, I know this is still early on, but what the hell. Come back to my place?"
"Sure!"
* * *
"OK, just let me slip out of these..."
"Wow, odd underwear. Are you Amish? Why do they have a lock on them?"
"Oh, don't worry, those are my Anti Rape knickers"
"You thought... I was going to rape you?"
"It was just a precaution. I don't need them now. But enough of that, let's get your clothes off too..."
"Because you thought I might rape you, but now you're pretty sure I won't rape you?"
"Come on, don't let this kill the moment, it's just a simple piece of protection"
"Nah, I'm out of here. I'll try not to rape anybody on the way home, but no promises eh, because you never know with us men! Good night" *slam*
Scenario 2:
"Victim is 22, female, appears to have been a pedestrian involved in the car crash. Heavy bleeding from the lower abdomen. Suspected femoral artery injury. Removing clothes to ascertain... wait, she's wearing some kind of chastity belt. No, we can't remove it, there are 512 possible combinations! OK, let's cut them off. What? The scissors don't work because the underwear is reinforced? Fuck, start guessing the combinations. Hurry up, she's bleeding out!"
Also, I'm usually a supporter of women taking ownership of their own personal safety. I think "tell the rapists not to rape!" is an overly idealistic position to take, and as long as we're dealing with the real world, people of both sexes should be taking commonsense precautions about their personal safety. But kevlar underwear? If a woman feels like she could get raped on a daily basis, my advice would be move to a better part of town. If I had to wear a bulletproof vest every day because my chances of getting shot were statistically high enough to warrant it, my first priority would be to get the hell out of there, not stock up on a range of discreet and fashionable ballistic vests.