do not see why you are so much bothered.... if your friend is happy with your underwear, let him be. Take a new set and put it on... that is what I would do
If you want a threesome, why should not you. Maybe he is bi and has a crush on you and is the undie thing just a way of coming close to you.
Would not be bothered too much with it. Next time, I would even propose to jack off together, and see how he reacts.