It was recently brought to my attention that sharing underwear is a universally unacceptable thing.  And I fully agree.  I'm ok with letting people borrow a shirt or two.  I'm more hesitant about pants and shorts, but I'll usually give in.  Socks and undershirts might be given out reluctantly with a "Hey, it's your problem" disclaimer. 

But when it comes to underwear, there's just no acceptable reason, socially, mentally, physically, or spiritually, to let another person wear your underwear.  It just can't happen.  The borrower would be virtually unable to make it through the day.  They might accidentally slip up and say, "Hi, I'm Bob.  I'm wearing my friend's underwear.  I mean..."  Or it might just happen to be the most hot and humid day of the year.  I don't need to elaborate on that point. 

The lender of the underwear would be equally at fault.  No matter how many times you wash them, no matter how much bleach and other industrial cleaners you use, they're still the underwear your friend borrowed.  Every time you wear them after that, you'll remember.  It's not one of those things you'll conveniently forget about like the dust bunnies you swept under the refrigerator or the cookie you accidentally dropped on the floor and ate.  I think you'd be so riddled with disgust and self-loathing that you'd be unable to continue being a functional human being.  You'd have frequent mental health breakdowns.  You'd start yelling at squirrels.  You'd be unable to enter normal society again until you burned those underwear in a bonfire and publicly renounced your utter moral failure. 

In conclusion, don't let people borrow your underwear, and don't be a borrower of other people's underwear.  It's just completely unacceptable.