I donate it to a thrift store.When I was reducing my underwear supply we tried to donate the first and largest batch and no one would take it. Much of it was in good condition. Do you just drop it in one of the collection bins?
The thrift stores said they sell stuff to a recycler. They take all textiles -- those they can't sell in the store get sold to the recyclers.
I think garments that contact intimate areas are not meant to be sold here as second-hand items, but that law is often honoured in the beach rather than the observance in local charity shops, especially for swimwear.
But if someone doesn't wear underwear, many items contact intimate parts and people wouldn't realize that.
I think no amount of washing and Clorox would induce most people to wear someone else's used underwear.Oh, I'm not saying they have thought these things through. :-)
But used jeans when the other person was ALWAYS commando. Well, we don't know that for certain, do we. Besides, the new owner probably wears underpants.