I don't understand the hesitancy to share underwear as long as it has been laundered. I wouldn't think twice about sharing a t-shirt or pants with a friend, why would I not share underwear if it was needed or if we were living together as in ArsenicAlive's case? I've often loaned friends swimwear when they want to go swimming at my place, and underwear seems to me to be no different.

I think for most it's just a mental hangup and a case of 'I've never done it'. This is often the case. Someone in the mists of time tells us 'it's wrong' and 'it shouldn't be done' and rather than looking at it logically, we accept it and use it as our own argument. My two cents worth.