We did this once before, as a topic; I was debating a similar donation of outgrown and barely worn underwear to my church's orphanage in the hills of the Dominican Republic, where any underwear is better than what they have. And where I assume that everybody's dirty underwear goes in the same laundry pot, so it's not like they aren't going to end up with someone else's "cooties" eventually.
I would say, wash with bleach, then fold and place in a zip-lock bag with a Post-it note inside (so you can read it through the plastic) that says "washed with bleach." And donate.
If they don't want it, they'll toss it.
I still remember reading on some forum once that a woman was really judgmental and aghast that someone bought her husband's used underwear at her yard sale. I thought, "If you think it's gross, why were you -selling- it?!?"
I grew up on hand-me-down underwear inside our family, and I don't care about imaginary cooties--just real-life germs. Which are killed by bleach.