In my experience models aren't terribly fond of wearing used underwear, regardless of whether it's been washed or not. Believe it or not, I've had this conversation before with models...and it's usually them complaining about some "creepy photographer that wanted me to put on some dirty, used panties". Now, even if they were exaggerating...you have to understand that the average person might have an aversion to wearing undergarments that they have no idea where they came from, the sanitary condition of them, and who wore them beforehand.
Pro tip: just give them to the model after the shoot and buy new for the next shoot. Besides, how many shots of the same set of panties do you need?