Question aimed at models really.
I have a small collection of outerwear that I have picked up with shoots in mind that models can wear. If used it is always washed or dry-cleaned.
I have noticed a number of photographers who state that they have large collections of lingerie that they want models to pose in. If that underwear is brand new, with price/fit tags still attached then fine but wearing secondhand underwear that you can't know has been washed seems unhygienic and, for the model, very unpleasant.
So, as a model, do you insist on using your own underwear or are you happy to step into someone else's used kit? If I ever was to buy lingerie specifically for a shoot I'd give it to the model (if they wanted it) or bin it at the end of the shoot.